Beat the fat, sugar and vanilla sugar with the whisk of the hand mixer until foamy. Separate the eggs, gradually stir the egg yolks into the fat and sugar mixture. Beat the egg whites until very stiff and add to the fat mass.
Mix flour and baking powder, sieve over it and carefully fold in. Place on a baking tray lined with baking paper and smooth down. Bake in a preheated oven (electric cooker: 200 °C/ gas: level 3) for about 15 minutes until golden brown.
Immediately after baking, turn out the dough sheet onto a tea towel sprinkled with sugar, remove baking paper. Soak gelatine in cold water. Heat lemon juice, zest and sugar until the sugar has dissolved.
Squeeze the gelatine and dissolve in the hot lemon juice. Put in a cool place. Whip cream until stiff and set aside approx. 1/3 for decoration. As soon as the lemon juice begins to set, stir in buttermilk and cream.
Spread the sponge cake first with the jam, except for 1 tablespoon for decorating. Then add the lemon cream and smooth it down. Roll it up with the help of a tea towel and let it set in the fridge for at least 2 hours.
Cut into about 12 pieces. Put the rest of the cream into a piping bag with a star-shaped spout and squirt it onto the sponge cake pieces as dots. Decorate with small lemon corners, jam and lemon balm leaves.