Preheat the oven to 180 °C (350 °F). Beat the eggs and sugar at maximum speed for 10 minutes. Prepare two 18 and 16 cm cake pans. Sift the flour over the egg mixture and stir it in gently with a spatula. Put slightly more than half of the batter in the larger cake pan, add cocoa to the rest of the mixture and put in the smaller cake pan. Level out both batters and bake the sponges for 13 minutes.
While baking the sponge, prepare the vanilla filling first, as it takes lon- ger to cool down. Bring 100 ml milk to a boil; mix 50 ml milk with sugar, vanilla, egg yolks and starch until smooth. When the milk boils, add the yolk mixture while stirring, then continue to cook until it thickens, stirring constantly. Set the prepared filling aside and cover with a plastic wrap in direct contact. Whip the cream and refrigerate. Soak gelatin sheets in cold water for 5 minutes.
Melt the chocolate and 50 g whipping cream in a double boiler, whip the rest of the cream (150 g) and refrigerate. Soak gelatin sheets in cold water for 5 minutes. When the chocolate has melted, use it to dissolve the wrung-out gelatin and cool until lukewarm. Then fold the whipped cream into the lukewarm chocolate with a spatula.
Put the white sponge on a stand, moisten and fit a cake ring. Coat with chocolate filling and level out the surface. Refrigerate the cake and finish the vanilla filling.
Gently wring the sheets of gelatin and dissolve them in the warm vanilla filling. Then mix with a hand mixer until smooth – mix until the filling is silky and has no lumps. Fold the whipped cream to the vanilla filling with a spatula.
Place the chocolate sponge cut to a 14 cm circle (4 cm less than the diameter of the cake) on top of the chocolate filling. Moisten the sponge and coat with vanilla filling – first fill in the space around the sponge. Level out the surface until smooth and place the cake in the refrigerator to set.
Remove the cake ring from the chilled cake and decorate as desired.