For the chocolate sponge cake:

1. Take the eggs out of the fridge ahead of time as they need to be at room temperature for the batter to work.
Preheat the oven to 350°F.
2. Spray three 8” cake pans with baking spray or else butter and flour them.
Beat eggs with sugar until they are very fluffy and pale yellow: it will take about 20 minutes using a stand mixer at medium/high speed. Sift flour with cocoa powder and add them to the egg mixture, a little at a time, folding the batter very gently with a wooden spoon.
3. Pour batter into prepared pans and bake for 20-30 minutes, or until a toothpick inserted into the center comes out clean. Turn off the oven but leave cakes inside for 5-10 minutes to allow them to cool down slowly. 
Remove cakes from the oven and let them cool for 10 more minutes. Then loosen around the edges with a knife and flip them upside down on a wire rack to cool completely.

For the filling:

Pastry cream:

1. In a large pan, combine milk and heavy cream and slowly heat the mixture without boiling it.
2. Meanwhile, in a separate bowl, beat egg yolks with sugar, add corn starch and mix well until combined. Slowly add the warm milk and cream mixture and keep stirring until you get a smooth batter without lumps.
3. Bring the cream back to the stovetop on low heat and whisk continuously until it becomes thicker and turns into custard. Be careful not to overcook it.
Transfer the pastry cream in a large bowl, cover with a plastic wrap and let cool completely.

Chantilly cream:

1. Whip cold cream in a cooled container. Once half-whipped, add powdered sugar. Continue blending with an electric whisk until the desired result is achieved. The more you whip the cream, the lighter and airier it will be at the end.
2. To make the diplomat cream, once the pastry cream has cooled off completely, incorporate the whipped cream into it, folding it in with a spatula from top to bottom to avoid losing volume. Carefully add fresh raspberries and mix until combined.

For the icing:

1. Using a stand mixer, beat butter until soft. Add half of the icing sugar and beat until smooth. Add remaining icing sugar and rum and beat the mixture until creamy and smooth, about 8-10 minutes.
2. You can make the icing in advance and keep it in the fridge until ready to use it. Remember to remove it from the refrigerator at least 2 hours before, allowing it to become soft and easier to spread.

To mount the cake:

1. Place one 8” sponge cake on top of a cake stand or serving tray and spread it with 1/2 of the berry diplomat cream. Gently layer the second cake on top and spread the remaining cream on it. Cover with the last round of sponge cake.
2. Using a spatula, carefully cover the whole cake with the softened buttercream.
3. If you’d like you could cover it with a layer of chocolate glaze and decorate it with candied orange slices and frosted red currants.