Separate eggs. Beat the egg whites until stiff, add salt, 1 packet of vanillin sugar and 75 g sugar. Stir in the egg yolks. Mix flour and baking powder, sift several portions onto the egg mixture and fold in. Place on a baking tray (32 x 39 cm) lined with baking paper, smooth down evenly and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes. Turn out onto a tea towel sprinkled with 2 tablespoons of sugar, carefully peel off the baking paper.
Roll up the plate with the cloth and let it cool down. For the filling, soak gelatine in cold water. Sort the blueberries, rinse and drain well. Pour into a tall mixing bowl and puree. Mix the ricotta, 1 packet of vanilla sugar and 75 g sugar with the whisks of the hand mixer. Stir in blueberries and soured milk. Squeeze the gelatine and melt it in a small pot at low heat. Stir 3 tablespoons of the mixture drop by drop into the gelatine and then stir into the remaining cream. Allow to draw until the cream begins to draw streets. Unroll the sponge, spread the cream evenly on the sponge, refrigerate again until the cream starts to set and roll up again carefully. Place in a cool place for about 3 hours.
Stir in blueberries and soured milk. Squeeze the gelatine and melt it in a small pot at low heat. Stir 3 tablespoons of the mixture drop by drop into the gelatine and then stir into the remaining cream. Allow to draw until the cream begins to draw streets. Unroll the sponge, spread the cream evenly on the sponge, refrigerate again until the cream starts to set and roll up again carefully. Place in a cool place for about 3 hours. Dust with icing sugar and cut into cake pieces. Decorate with blueberries and lemon balm
5 hours waiting time