In the words of Mary Berry, this bake is sheer perfection.
For the biscuit layers:
2 tbsp corn syrup
Red food coloring
3/4 cup flour
1 1/2 tbsp cocoa powder
1/2 cup powdered sugar
1 tsp baking powder
Large pinch of salt
1/3 cup butter, melted
1/2 cup dark chocolate, melted
For the Bavarian cream:
10 sheets leaf gelatin
2 1/2 cups milk
8 egg yolks
1/2 cup sugar
1 tsp vanilla extract
1 3/4 cups heavy cream
Make the biscuit layers: Preheat the oven to 350 degrees. Line two large baking trays with parchment.
Whisk the eggs for 3 minutes until fluffy. Add the syrup and continue whisking for 1 minute, then add in enough food coloring to give a deep red color. Sift in the flour, cocoa powder, powdered sugar, baking powder and salt, then whisk together. Add in the melted butter and mix until just combined. Divide the mixture between the two trays and spread it out smoothly across the paper.
Put the melted chocolate into a piping bag and snip off the very end. Pipe straight lines of chocolate across the biscuit batter, 1/2 inch apart. Use a cocktail stick to drag alternating lines across the chocolate to create a feathered pattern. Reserve the remaining chocolate.
Bake for 12 minutes until browning around the edges and cooked on top. Leave to cool on the tray for 10 minutes, then transfer the biscuits on their paper to a wire rack to cool completely.
Make the Bavarian cream: Put 7 gelatin sheets into one bowl, and 3 into another. Cover them all with cold water and set aside.
Bring the milk to a gentle simmer in a large saucepan, then remove from heat. In a large mixing bowl, add the egg yolks, sugar and vanilla. Mix well, then pour the warm liquid over the eggs, whisking immediately to combine and avoid lumps. Pour the mixture back into the saucepan and heat gently, stirring constantly until thick, then remove from heat.
Pour one-fourth of the custard into a separate bowl. Stir in the reserved melted chocolate and enough food coloring to give a deep red color. Squeeze the water from the 3 gelatin sheets, then mix them through the still-warm chocolate cream until fully dissolved. Cover the top of the chocolate cream with plastic wrap to stop a skin developing, then allow to come to room temperature.
Squeeze the liquid from the 7 gelatin leaves and mix them into the larger vanilla cream (it should still be warm). Cover the top with plastic wrap and leave to cool.
While the custards are cooling, whisk the heavy cream to soft peaks. Place one-fourth into a separate bowl.
Assembling the cake: While the custards are cooling, cut out the first biscuit layer to fit into the bottom of an 8-inch cake pan. Line the base of the pan with parchment, then place the biscuit at the bottom, pattern-side down.
Once the chocolate custard is cool, fold the smaller amount of whipped cream in, then pour it into the smaller cake mold and place in freezer to set.
Once the vanilla custard has cooled, fold in the remaining whipped cream. Pour one-third of this mixture into the cake pan on top of the first biscuit layer. Put the pan into the freezer for 15 minutes. When the vanilla layer has been in the freezer for 15 minutes, take both pans out of the freezer. Carefully remove the chocolate layer from the pan (running a warm knife inside will help). Carefully place the chocolate layer right in the middle of the vanilla. Pour the remaining vanilla cream over the top and smooth it over, then place the remaining biscuit on top and push it down so it’s flat against the top of the cream.
Chill the cake for 3 hours in the fridge, or overnight, then slice and serve.