For the sponge cake separate the eggs. With a hand mixer, mix the egg yolks along with 50 g sugar, vanilla sugar, and lemon zest to a creamy consistency. Beat the egg whites with salt and the rest of the sugar until fluffy. Next, carefully fold the resulting fluff and then the flour into the egg yolk mixture. Wrap a cake ring (26 cm diameter) in parchment paper. Fill the mixture into the ring and bake for 40 minutes at 160 °C (with fan on). Remove from the ring once cooled.

For the diplomat's cream soak the gelatine in cold water. Heat the milk along with 75 g cream, the sugar, and the vanilla bean pulp. Stir together cornflour and egg yolks, stir this mixture into the milk, and simmer while whisking vigorously. Squeeze out the gelatine and melt it in. Beat the mixture over ice water until chilled. Work in the butter in pieces. Stir the rest of the cream until it thickens a bit and then fold it in.

For the fluff heat the two egg whites along with the sugar to 60 °C over steam. Beat until stiff, remove from the steam, and stir until cooled.
Slice the baked sponge cake into three equal slabs. Return the first slab to the cake ring and sprinkle on some rum. Spread half of the diplomat’s cream onto the cake slab and repeat the process. Allow the punch cake to cool and grow firm in the refrigerator.

Before serving place the cake on a platter, separate it from the cake ring with a knife, and remove the ring. To decorate, fill the fluff into a piping bag and pipe on the desired pattern using a smooth, round nozzle. Brown protruding bits of fluff with a Bunsen burner.