Separate the eggs for the sponge cake. Beat the egg whites and 2 tablespoons of cold water until stiff. Add salt and 100 g sugar. Stir in egg yolks. Mix starch, flour and baking powder. Sift the flour mixture over the egg mixture and fold in
Smooth out 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: see manufacturer) for approx. 10 minutes. Remove and turn out onto a tea towel sprinkled with sugar. Remove the baking paper. Roll up the sponge cake with the tea towel and let it cool down
Soak 6 and 2 gelatine leaves separately in cold water. Mix yoghurt, vanillin sugar and 40 g sugar. Warm the berry mix and 50 g sugar over low heat. Squeeze 2 sheets of gelatine, add and mix well. Remove from the heat, pour into a bowl and let cool
Squeeze 6 sheets of gelatine. Melt in a small saucepan on low heat. Stir 2-3 tbsp. of the yoghurt cream drop by drop into the gelatine. Then run into the remaining yoghurt. Chill for 5-8 minutes until the mixture begins to gel. Whip the cream until stiff and fold in portions
Add the berry mixture in streaks. Unroll the sponge cake, spread evenly with the cream, leaving an approx. 1.5 cm wide free edge on the upper long side
Roll up the sponge cake from the lower long side. Chill for about 3 hours. Then dust with icing sugar. Cut into 10 cake pieces. Serve with blueberries and lemon balm, again dusted with a touch of icing sugar.
waiting time approx. 3 3/4 hours