Separate the eggs for the sponge cake. Beat the egg whites with the whisk of the hand mixer until stiff, add 1 packet of vanilla sugar, salt and 100 g sugar. Stir in the egg yolks one after the other. Mix flour, starch and baking powder, sift into several portions onto the egg mixture and fold in. Spread evenly on a baking tray (32 x 39 cm) lined with baking paper. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes.
Then turn onto a damp tea towel evenly sprinkled with 75 g sugar and carefully remove the baking paper. Turn over protruding sides of the tea towel. Let the sponge cake cool down. In the meantime, soak gelatine in cold water. Mix yoghurt, 150 g sugar, 1 sachet vanilla sugar, lemon peel and lemon juice. Whip the cream until stiff. Squeeze the gelatine and melt it in a small pot at low heat. Stir in 4-5 tablespoons of yoghurt mass drop by drop and then stir into the rest of the yoghurt mass. Chill for 2-4 minutes until the mass begins to gel. Fold in cream in portions. Spread evenly on the sponge cake.
Whip the cream until stiff. Squeeze the gelatine and melt it in a small pot at low heat. Stir in 4-5 tablespoons of yoghurt mass drop by drop and then stir into the rest of the yoghurt mass. Chill for 2-4 minutes until the mass begins to gel. Fold in cream in portions. Spread evenly on the sponge cake. Roll up from the long side and refrigerate for about 3 hours. Then cut the roll into cake pieces and decorate with lemon wedges