Grease and line a 33x23cm/13x9in Swiss roll tin with baking parchment. Cut a sheet of baking silicone to fit the base of the Swiss roll tin. Line a baking tray with baking parchment and lay the silicone sheet on top.
For the decorative paste, cream the butter and icing sugar together until light and fluffy. Gradually add the egg whites, beating continuously. Fold in the flour and cocoa powder until combined.
Spread the paste thinly and evenly over the silicon mat using a palette knife. Drag a pastry comb diagonally through the paste to form a pinstripe pattern, scraping off the excess mixture after each drag. Place the paste in the freezer for 15 minutes.
For the joconde sponge, preheat the oven to 220C/200C(fan)/Gas 7.
Whisk the egg whites in a bowl until stiff peaks form when the whisk is removed. Add the caster sugar, one teaspoon at a time, whisking between each addition to make a glossy meringue. Cover with cling film and set aside.
Tip the ground almonds and icing sugar into the bowl of a freestanding mixer and add the whole eggs. Whisk together for 3-5 minutes, or until doubled in volume. Fold in the flour, and then gently fold in the meringue in three separate batches. Pour the melted butter down the side of the bowl and fold in, until incorporated.
Remove the silicone sheet with the decorative paste from the freezer and place into the base of the lined Swiss roll tin.
Tip the joconde sponge mixture over the pinstripe paste and level the surface with a palette knife. Bake for 5-7 minutes, or until pale, golden-brown and springy to touch.
Place a sheet of baking parchment over a cooling rack and turn the cake out onto it. Peel off the baking parchment and silicon sheet from the base of the cake and leave to cool completely.
Line the sides of 6 x 6.5cm/2½in chefs’ rings with baking parchment. Cut a rectangular template measuring 17x5cm/6½x2in, from a piece of parchment paper.
When the sponge is cooled, trim the edges and cut out six rectangles of sponge using the template. Wrap the cake strips inside the cake rings, stripey side facing out. Once wrapped inside the rings, gently push and press the ends to meet together to make a seamless cake. You want the joconde to fit very tightly pressed up to the sides of the cake rings.
Using a small 4cm/1½in round cutter, cut out small circles from the cake trimmings and push them into the base of the cake rings.
For the dark chocolate mousse, place the chocolate in a bowl set over a pan of simmering water (ensure the bottom of the bowl does not touch the water). Melt slowly and do not allow the chocolate become too hot.
Stir in the butter and mix until combined. Add the egg yolks and brandy and set aside to cool a little.
In a bowl, whisk the egg whites until stiff peaks form when the whisk is removed. Fold into the chocolate mixture until completely incorporated.
Whip the cream until soft peaks form when the whisk is removed. Fold into the chocolate and egg white mixture.
Spoon the mousse into a disposable piping bag and pipe into the cake lined rings, until the mousse comes three-quarters of the way up the sponge. Chill them in the freezer while you make the white chocolate mousse.
For the white chocolate mousse, place the chocolate in a bowl set over a pan of simmering water (do not allow the bottom of the bowl to touch the water). Melt very gently, but do not allow the chocolate to become too hot.
In a bowl, whisk the egg whites until stiff peaks form when the whisk is removed. Fold into the white chocolate mixture.
In a bowl, whip the cream until soft peaks form when the whisk is removed and fold into the white chocolate mixture.
Spoon the white chocolate mousse into a disposable piping bag and pipe on top of the chocolate mousse until it reaches the top of the cake rings. Chill in the fridge until set.
Once set, turn out of the cake rings and place on serving plates. Top with the raspberries and blueberries and dust with icing sugar.
