Baking challenge: chocolate hazelnut mousse cake. Oh yes

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 signature challenge for week seven (patisserie week) of series two: a layered mousse cake.

Chocolate hazelnut mousse cake with hazelnut crunch base
Chocolate hazelnut mousse cake with hazelnut crunch base

Think a crisp, rice-cereal-marshmallow-and-hazenut base, bound together with melted chocolate. Think a silken, rich yet feather-light mousse that melts in the mouth like snow, leaving behind the impression of chocolate and hazelnut. Think a soft, buttery cake which adds another layer of contrast to the delicate mousse and robust, crunchy base. That is this cake and it is absolutely stunning, and completely worth it.

Lest you think the above is sheer hyperbolic food porn, I assure you that everyone I served this to thought it was utterly divine. It’s a special cake, rather than an everyday, cut-and-come-again cake, that would be perfectly well-suited to being served as a dessert at the end of a lovely dinner party (perhaps served with some of the Frangelico that lends it its hazelnutty, smoky flavour).

Slice of chocolate and hazelnut mousse cake, in layers
Slice of chocolate and hazelnut mousse cake, in layers

The baking ladies of series two of GBBO (who were tasked to make mousse cakes) almost all used gelatine in their mousse, but this one relies on just the cream for aeration and lift. This is of course what gives the mousse its delicate, melting quality, but also means that it isn’t as structurally strong as one reinforced with gelatine, hence lengthy chilling is essential. The mousse is prone to melting, as well, because of the soft texture and structure, so chill it between serving.

It's just absolutely got to be done!
It’s just absolutely got to be done!

The recipe I used is adapted from a Bon Appetit recipe which I found on Epicurious. Apart from a few additions and adaptations based on the reviews (for example I added marshmallows to the base because a lot of the reviews said the base was too hard) and adapting it for ingredients easily found in the UK, I have also tried to streamline some steps as it was quite a fussy and fiddly recipe, which required odd things such as baking the cake in a particular-sized tin and then trimming it to fit another size of tin. Just say no. I didn’t do it and I didn’t need to.

For the hazelnut flavour, this mousse cake relies on a good dose of Frangelico – hazelnut liqueur – in addition to the use of hazelnuts in the base. You can occasionally find hazelnut liqueur which is not Frangelico, but it’s not easy and the price is around the same. It’s definitely quite a niche product and I know how wearying it can be to be guided towards an ingredient which is expensive, sometimes difficult to find, and not very versatile. In my defence, though, this cake is really delicious and very special. If you want to try it but are desperate not to buy in the Frangelico, dark rum could work.

Hazelnut and chocolate mousse cake with chocolate hazelnut crunch base
Adapted from Bon Appetit magazine, via Epicurious

Chocolate cake layer

  • 60g unsalted butter
  • 1 tsp vanilla extract
  • 35g plain flour, sifted
  • 35g cocoa powder, sifted
  • 4 large eggs
  • 120g sugar
  • 2 TBS Frangelico liqueur
  1. Preheat oven to 180C. Line a 23cm cake tin with baking paper.
  2. Melt the butter in a saucepan over a medium heat and remove from the heat once melted. Carefully spoon off the foam from the top of the butter and discard. Carefully pour or spoon the clear yellow butter into a small bowl, leaving the water and milk solids in the pan. Add the vanilla to the clarified butter and set it aside.
  3. Sift the flour and coca together three times into a medium bowl.
  4. In a large heatproof bowl, whisk the eggs and sugar together. Set the mixture over a pan of barely simmering water and whisk constantly until the mixture is lukewarm, which will take around 2 minutes. Remove the bowl from the water.
  5. To keep the clarified butter warm, place the bowl of clarified butter in the saucepan of hot water (you may need to pour some of the water off and it’s important the water does not mix into the clarified butter).
  6. Using an electric mixer (handheld is fine, it’s what I have), beat the egg mixture until it is cool and tripled in volume – around 5 minutes. Sift a third of the cocoa-flour mixture over the egg mixture and gently fold it in with a flexible spatula. Fold in the remaining flour and coca in two additions (i.e. a third each time).
  7.  Fold about 250ml (i.e. an American cup measure) of the cake batter into the warm clarified butter until incorporated. Using a spatula, fold this mixture of cake batter and clarified butter into the remaining cake batter. Add the two tablespoons of Frangelico to the batter and incorporate.
  8. Transfer the cake mix to the prepared pan. Bake for around 30 minutes, until a tester inserted into the centre of the cake comes out clean, and the top springs back slightly when pressed. Allow to cool completely in the pan on a rack. Once cool, run a palette knife between the cake and the sides to loosen if necessary (my cake shrank back from the sides, so it wasn’t required) and invert the cake onto the cooling rack; remove the baking paper. If necessary – if the cake has domed – trim the top of the cake horizontally using a long serrated knife, so that you have a flat surface.

Hazelnut crunch base layer

  • 140g milk chocolate
  • 1 TBS Frangelico
  • 60g crisp rice cereal
  • 100g hazelnuts
  • 140g marshmallows
  1. Line the bottom of a 23cm springform pan (yes, another one) with baking paper
  2. Preheat the oven to 180C (or do it immediately before or after baking the cake, above) and toast the hazelnuts for 6-8 minutes, until toasted and golden. Remove from the oven and place the hazelnuts in the centre of a clean tea towel. Bring the corners of the tea towel together and rub the hazelnuts between the tea towel, being careful not to lose too many. This removes the papery outer husk of the hazelnuts, though don’t get stressed about the occasional fleck. Once done, either chop the hazelnuts or crush them in a pestle and mortar (I did the latter as the pinging of nuts around the kitchen as I try and chop them drives me insane; it’s especially difficult to chop up round nuts).
  3. Melt the chocolate in a bowl set over a saucepan of barely simmering water. Once melted, remove from the heat. Add the Frangelico and the marshmallows and stir in; they will melt in the residual heat. Add in the nuts and rice cereal and stir until evenly distributed. Spread the crunch base evenly over the base of the pan. It will be very sticky.
  4. Place the cooled cake on top of the crunch layer, pressing lightly so that they adhere. Chill for about an hour, until the crunch base layer is firm.

Chocolate and hazelnut mousse

  • 285g milk chocolate
  • 300ml double cream
  • 1/8 tsp salt
  • 2 TBS water
  • 1 TBS Frangelico
  1. More melting of chocolate here; melt in a heatproof bowl set over a saucepan of barely simmering water.
  2. Beat the cream and salt in another bowl until medium-firm peaks form. The original recipe recommended using a mixer for this but I don’t as the line between peaks and butter is very fine indeed when it comes to using a mixer.
  3. Mix the two tablespoons of water and the Frangelico into the melted chocolate, whisking it to incorporate. It will look seized and stiff and awful; just keep whisking until it gets smooth again. Pour the whipped cream over the chocolate mixture and fold until just incorporated. You should expect the mousse to be quite soft.

To assemble

  • 3 TBS Frangelico
  1. Brush the top of the chilled cake with the Frangelico. Spread the mousse over the cake in the pan; cover and chill it overnight.
  2. The next day, run a palette knife between the cake and pan sides to loosen it. Carefully unlock the spring clip and remove the pan sides from the cake. If you think it looks very resistant, take a kitchen towel, soak it in hot water, wring it out and wrap the hot, wet towel around the pan sides for 30 seconds before attempting to loosen.
  3. If necessary, smooth the top and sides of the mousse with a palette knife. Place the assembled cake onto a serving dish.



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