Separate the eggs for the sponge cake. Beat the egg whites with 1 pinch of salt until stiff, adding 75 g sugar and vanilla sugar. Stir in the egg yolks one by one. Sift flour, baking powder and 1 tbsp. cocoa onto the egg mixture and fold in carefully.
Spread the mixture on a baking tray lined with baking paper (approx. 35 x 40 cm). Bake in a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 10 minutes. Sprinkle a clean tea towel with 2 tbsp. sugar. Fold the sponge cake onto it and remove the baking paper. Loosely roll up the warm sponge cake plate together with the cloth and let it cool down.
For the filling, soak the gelatine in cold water. Set aside 4 strawberries for decoration, wash, clean and finely dice the remaining berries. Mix yoghurt, lemon juice and 125 g sugar. Squeeze the gelatine and dissolve at low heat. First stir 2-3 tbsp. yoghurt into the gelatine, then stir everything into the remaining yoghurt. Fold in the strawberry cubes. Chill for about 40 minutes until the cream starts to gel. Whip the cream until stiff. Approx. 3⁄4 Fold the cream into the yoghurt.
Carefully unroll the sponge cake. Spread the yoghurt cream on top, leaving about 2 cm of edge free. Roll up the sponge cake again. Chill for about 4 hours.
Wash, clean and dice the strawberries that have been set aside. Pour the rest of the cream into a piping bag with a star-shaped spout. Dust sponge roll with 1 tbsp. cocoa and decorate with cream, berries and jam.
Carefully lift the tea towel steeply on one long side. This way the plate rolls up almost by itself.