Preheat your oven to 350°F (175°C). Grease a 9-inch round cake tin and line the base with parchment paper.

Melt the butter and let it cool slightly. In a mixing bowl, whisk the eggs and sugar together until pale and fluffy. Sift the flour, cocoa powder, cornstarch, baking powder, and salt into the mixture. Fold gently, ensuring there are no lumps. Add the melted butter and mix until just combined.

Pour the batter into the prepared tin and bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the sponge cool in the tin for 10 minutes, then transfer it to a wire rack to cool completely.

Dissolve the powdered gelatin in the warm water and set aside to bloom.

Place the dark chocolate in a heatproof bowl and melt it over simmering water. Stir in the brandy and remove from heat. Let it cool slightly before folding in the egg yolks.

Whip the cream to soft peaks and fold it gently into the chocolate mixture. In a separate bowl, whisk the egg whites until stiff, then fold them into the mousse along with the prepared gelatin.

Slice the sponge horizontally into two layers. Place one layer on a serving plate and spread half of the mousse evenly over the top. Add the second layer and cover the entire cake with the remaining mousse. Chill the cake for at least 4 hours, or until set.

Melt the dark and white chocolate separately. Spread each onto a parchment-lined baking sheet and allow it to set. Once firm, break the chocolate into shards.

Spread whipped cream over the top of the cake and decorate with the chocolate shards. Dust with confectioners’ sugar for a final touch.