Separate eggs. Beat the egg whites and 4 tablespoons of water until stiff. Gradually add 150 g sugar and vanillin sugar. Then stir in the egg yolks one by one. Mix flour, cornstarch and baking powder and sieve onto the egg foam. Carefully fold in.
Pour the sponge mixture onto a baking tray (32 x 39 cm) lined with baking paper, smooth it down. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: unsuitable/ gas: level 3) for 10-12 minutes. Turn the sponge onto a damp tea towel sprinkled with 2 tablespoons of sugar, carefully peel off the baking paper. Let the sponge cake cool down. In the meantime, roughly chop the chocolate blocks and melt them on a hot water bath, let them cool down a little. Sift icing sugar. Beat the fat and icing sugar until creamy with the whisk of the hand mixer. Carefully fold the chocolate and rum into the buttercream. Spread 2/3 of the cream on the sponge cake and roll it up from the long side. Spread the rest of the buttercream all around. Make a tree trunk pattern with a knife. Cut a piece from the roll and place it on the main trunk as a branch.
Beat the fat and icing sugar until creamy with the whisk of the hand mixer. Carefully fold the chocolate and rum into the buttercream. Spread 2/3 of the cream on the sponge cake and roll it up from the long side. Spread the rest of the buttercream all around. Make a tree trunk pattern with a knife. Cut a piece from the roll and place it on the main trunk as a branch. Chill the tree trunk for about 1 hour. Before serving, dust with icing sugar if necessary
Waiting time approx. 1 hour