Rachel Allen’s chocolate mousse layer cake combines four layers of chocolate sponge sandwiched together with rich chocolate mousse.
Preheat the oven to 180C/160C fan/gas 4. Butter the sides of two 23cm cake tins with 6cm sides and dust with flour, then line the base of each tin with a disc of baking parchment.
Melt the butter and set aside for 5-10 minutes to cool until tepid. Place the eggs and sugar in a large bowl and using an electric whisk, whisk for 6-8 minutes or until the mixture is light and thick. To test if it’s thick enough, lift the whisk out of the mixture and draw a figure of eight in the bowl with the batter that’s left on the whisk - the ‘8’ should remain visible on the surface for a couple of seconds.
Sift in the flour, cocoa powder, baking powder and salt and fold in gently but thoroughly, then pour in the vanilla extract and the melted butter and fold in again.
Quickly divide the batter between the prepared cake tins before it has a chance to lose any volume, then bake for 30-35 minutes, or until the mixture springs back when lightly pressed with a finger and a skewer inserted into the centre of each cake comes out clean.
Remove from the oven and allow to cool in the tins for 5 minutes, then loosen around the edges of each tin using a small, sharp knife and carefully remove the cake before transferring to a wire rack to cool down completely.
For the mousse: Place the chocolate in a heatproof bowl and set over a saucepan of simmering water. Leave just until melted, stirring occasionally, then remove from the heat. Beat in the egg yolks one at a time, then beat in the butter, spirits or orange liqueur or vanilla extract and keep beating until smooth.
Set aside to cool while you whisk the egg whites. In a spotlessly clean bowl, whisk the egg whites until they form stiff peaks. Fold in a quarter of the chocolate mixture, then carefully fold in the rest just until mixed. Set aside and leave to cool at room temperature – not in the fridge, otherwise the mousse will harden and become difficult to spread.
To assemble the cake, first use a bread knife to carefully slice each cake in half horizontally. Place the bottom half of one of the cakes, cut side up, on a plate or cake stand. Spread over some of the mousse in a layer about 5mm thick, then sandwich with the other half of the cake, placing it cut side down.
Spread over more of the mousse, just as thickly, then sandwich with the bottom half of the second cake, cut side up. Repeat for the next layer until you have four layers of cake sandwiched together with three layers of mousse. Cover the top and sides of the assembled cake with the remaining mousse, smoothing it over with a palette knife.