Baking challenge: hidden design rainbow zebra cake

Baking Advent: celebrating the festive season with a different daily baked good.

Eat the rainbow

This post is part of my personal challenge to bake my way through all the challenges of the Great British Bake Off. The challenge below is the showstopper challenge for week one (cake week) of series three: a hidden design cake.

Although I like to bake and cook, I am not the person who brings in homemade goods to the office. It happens occasionally, but when I bake it’s usually for me and my boyfriend, or friends, and I don’t always have much time to bake extra for work. Shortly after starting my first big grown-up job post-university, I coincidentally read a few articles which warned that, in fact, women should avoid bringing baked goods to the office to avoid being written off as domestic rather than professional, but I don’t work in that kind of high-level, male-dominated, hyper-corporate environment where those issues would be at play. And I do have friends who work in tough corporate jobs with 18-hour days and billable hours and clients and liability issues and they bring in baked goods nonetheless, because their work speaks for itself in terms of their professional abilities. That, to me, seems like a good sign in the context of debates about women’s roles in the workplace. My issue is less office politics, more lack of time.

Chocolate cream cheese frosting
How to hide a rainbow surprise (but still hint at the promise of colour)

A while ago, however, I did bring in a homemade cake for a colleague’s leaving do. I work in quite a small organisation, and I’ve said goodbye to many colleagues over the last five years, as they find opportunities outside of our tiny, very specialist institution. My colleagues are immensely talented, lovely people, and it’s always sad to see them go. Everyone seemed surprised and delighted by the cake – but I think it would be hard to avoid being charmed by it, with its bright colours revealed in every slice, belying a velvety-smooth but traditional-looking exterior of plain, creamy chocolate cream cheese icing.

The first showstopper challenge of series three of the Great British Bake-Off required the bakers to make a hidden-design cake – that is, a cake which, when cut into, reveals a pattern or image cunningly baked or carved into the centre. There are three basic ways of making a hidden-design cake: chiselling out the centre of a baked cake and replacing it with a filling, modelling chocolate, more baked cake or similar; pre-baking sponge cake into slices and pouring cake batter over and re-baking; or, thirdly, creating a design using cake batter in the pan entirely before baking.

Rainbow zebra cake inside
A hasty ‘action shot’ of the cut cake at the party

As I’m really not very good at fancy designing and decorating, I opted for the third method; I also thought that avoiding fiddling around with pre-baking would avoid the possibility of ending up with a dry cake. Also, by choosing this method, I managed to put a cake together that looked exciting but is actually do-able on a weeknight (I did it, so I know it is possible!).

I liked the idea of a zebra or giraffe cake, and when I saw the rainbow zebra cake on the Youtube channel My Cupcake Addiction, I decided to make it, excited by the combination of a crazy colour scheme but also a fairly simple technique. I followed the basic instructions from that video, although I didn’t use a boxed cake mix; instead, I opted for a plain yet buttery sponge with sufficient structure and density to support the addition of plenty of colouring paste. I covered and filled these cakes with a rich, creamy chocolate cream cheese icing. Because this recipe replaces some of the usual icing sugar used to stiffen the cream cheese frosting with cocoa powder, it’s less sweet than many cream cheese frostings and also darkly delicious. I couldn’t resist then flinging the cake with some coloured dragees I had in the cupboard, to give a hint as to the colourful inside.

Slice of rainbow zebra cake

Of course, the first design cake I ever made has come with a learning curve. I would use more cake batter than I did the first time around, because I ran a little low, which resulted in the layers of colour merging rather than being sharply delineated, as I had to scrape and scrimp towards the end. I think, if you want to bake a zebra cake of your own, that an additional half-portion of batter would work well (I have the recipe below, both as I baked it and my suggested measurements for a greater volume of cake batter).

Secondly, I think the pictures illustrate well the difference between using professional food colouring paste and colouring paste aimed at domestic consumers. In my cake, the black and blue colouring came from Lakeland, and once baked the colours are vibrant and true; the green and pink colours were from Dr Oetker (picked up at the supermarket) and, while the colour looked vibrant when the cake was raw, they baked up much paler.

Black, blue, pink and green food colouring

There are quite a few steps to making this cake – although none of them are hard in and of themselves – so I have, unusually, included some photos in the instructions below.

Rainbow zebra cake

Note: As I say above, if making this again I would use more batter than used below in the main recipe. The below outlines how I actually did it, but if you want to make it with more batter from the get-go, use the following measurements:

  • 300g butter
  • 300g caster sugar
  • 5 eggs
  • 2.5 tsp vanilla extract
  • 300 plain flour
  • 3 tsp baking powder
  • bigger pinch salt
  • Food colouring paste, as below

You could also, of course, simply double the batter recipe below.

For the cake
The vanilla sponge recipe is adapted from Frances Quinn’s Quinntessential Baking, although it is a standard sponge mix.

Note: Ordinarily, you would add some milk or warm water to bring a sponge to what is known as ‘dropping consistency’, i.e. so that the batter is still fairly thick but drops off a spoon when gently shaken. However, for this particular recipe, you will be adding quite a lot of food colouring to the batter, and the addition of the liquid should be enough to bring it to dropping consistency. This is especially true if you are not using a colouring paste but rather a liquid, though I urge you to use a paste, as the batter needs to be fairly firm for it to stand up to the dolloping that’s going to commence, and adding enough liquid food colouring to colour the batter would disrupt the proportions and make the cake layers sloppy.



  • 200g butter, softened
  • 200g caster sugar
  • 4 eggs
  • 2 tsp vanilla extract
  • 200g plain flour
  • 2 tsp baking powder
  • pinch of salt
  • Black food colouring, ideally a paste
  • Three additional food colouring pastes of your choice – I used blue, pink and green


  • Preheat the oven to 180C.
  • Grease two sandwich pan trays. Cut four circles of baking paper out so that they will fitinside the cake pan, and line the bottom of the sandwich pans with one circle of baking paper each. Reserve the remaining two circles.
  • Beat the butter and sugar together until very light, pale and creamy, ideally using a hand-held electric whisk – it should take between 5-10 minutes. The sugar should be dissolved into the butter.
  • Beat together the eggs and vanilla extract and gradually add the egg to the butter mixture, beating well after each addition to avoid curdling. If the mixture does start to split, add a spoonful of the flour and continue.
  • Once the egg has been added and thoroughly beaten in, sift the flour, baking powder and salt into the egg mixture in two to three batches, folding it in each time.
  • Divide up the batter between four separate bowls as follows: about half for the black colour, and the remaining half among the three other colours. This is because every second stripe will be black, so you need more of the black food colouring.
  • Mix in the black food colouring into the half of the cake batter set aside for this colour, and colour the remaining batches of cake batter in the colours of your choice. Make sure to keep the spoons used for mixing in the colours separate!
  • Take your lined sandwich pans and start by placing a soup spoon-sized dollop of black cake batter in the centre of each, keeping the black food colouring as centred and circular as possible.
  • Dollop of black batterTake your first coloured batter and place a soup spoon-sized dollop of this coloured batter over the circle of black batter (don’t forget to do each step for each pan). Place another dollop of the black batter ontop of the first colour, then place a dollop of your second coloured batter over this dollop of black batter. Repeat with the next colour.

First colour of batter on top of black

  • Every four-five layers, lift up your sandwich pans and tap them gently against the counter to flatten them out. Continue layering up the batter, remembering to dollop the black batter between each layer of coloured batter. Finish with a final dollop of black batter in the centre of the cake when all the coloured batter has been used.
  • When all the coloured batter has been used and you are ready to bake, pick up the sandwich pans and tap against the counter again to flatten out all the batter and have the cake layers as evenly as possible
  • Take the two baking paper circles you set aside earlier and gently press the circle of baking paper on top of the batter. This will facilitate the cakes rising evenly in the oven.
Baking paper over coloured batter
Baking paper over coloured batter
  • Bake the cakes for 20-25 minutes, until a cake tester inserted into the centre comes out clean and the top of the cakes spring back when lightly pushed. You can gently ease off the baking paper to test the cakes with the skewer. Be careful to ensure the cake doesn’t become too brown as this will dull the colours.
  • Allow to cool in the tins for about 15 minutes before turning out to a wire rack to cool completely.

For the icing
Using the method from The Hummingbird Bakery Cookbook – it produces a smooth, creamy yet thick icing


  • 100g unsalted butter, room temperature/softened
  • 500g icing sugar, sifted
  • 100g plain cocoa powder, sifted
  • 250g cream cheese, cold


  • Using an electric mixer, beat together the icing sugar, cocoa and butter on medium-slow speed until the ingredients are well-combined.
  • Add the cream cheese all in one go and beat until incorporated, on the same speed.
  • Turn the mixer to medium-high and continue beating for at least five minutes, until the frosting is light and fluffy – be careful not to overbeat as this will make the icing runny.

To assemble

  • Place the cakes on a flat surface and, using a serrated knife, carefully trim the top of the cake off so that the cakes have a level surface and reveal the bright colours. underneath. Go carefully; you don’t need to trim off too much.
  • On the cake board or serving plate you will use to serve the cake, place one of the sandwich cakes trimmed side down, so that the flat side is on the top.
  • Dollop about a quarter to a third of the chocolate cream cheese icing on top of the cake and smooth out to the side using a spatula, palette knife or spoon, leaving a ring of 0.5cm around the edges clear.

First application of chocolate cream cheese icing on bottom layer

  • Place the second sandwich cake on top of the icing, trimmed side down so that, again, the flat side is at the top. The pressure of the cake on top of the icing should help it move out to the sides, covering the gap at the edges. Don’t worry if you’ve put too much icing and it goes down the sides a bit.
  • To have a smooth, tidy icing without any messy crumbs in it, you’ll want to give it a crumb coat. Start by smoothing out any excess icing which has come out of the sides of the cake around the cake using a palette knife or ordinary knife.
  • Using a clean spoon, dollop a little less than half of the icing on top of the cake and smooth it over the top. Then, using your palette knife, gently smooth the icing down the sides of the cake to create a thin layer of icing across the entire surface of the cake. Then gently push the palette knife along the sides of the cake so that the first layer of icing is uniform and fairly smooth across the entire surface of the cake – top and sides. This layer will trap any loose crumbs on the surface of the cake. Chill the cake for 20 minutes or so.
  • Remove the chilled cake from the fridge and, using a cleaned palette knife, take the remaining icing and smooth it over the cooled first layer of icing. I tend to go for a swirled effect rather than a perfectly smooth one. The crumbs caught in the first layer of icing will now not transfer to the surface of the cake.
  • You can now throw on any edible decorations you like, or leave it elegantly and deceptively plain, as you prefer.



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