Beat the eggs and sugar for about 10 minutes until thick and creamy. Sift flour and cornstarch over it. Add lemon zest and fold in carefully. Spread the dough on a baking tray lined with baking paper.
Bake in the preheated oven (electric oven: 200°C/ gas: level 3) for 10-12 minutes. Immediately turn onto a tea towel sprinkled with sugar and remove the baking paper. Cover with a damp tea towel and allow to cool.
Select the blueberries, wash and drain well. Puree 50 g blueberries. Soak the gelatine. Mix quark, pureed blueberries, 60 g sugar and vanillin sugar until smooth. Squeeze the gelatine, dissolve and stir into the quark.
Cool briefly. Whip 200 g cream until stiff. Put some blueberries aside for decoration. Stir the remaining blueberries and cream into the quark. Spread on the sponge cake and roll up carefully with the help of the tea towel.
Put in a cool place for about 1 hour. Whip remaining cream and sugar until stiff. Spread the sponge roll with cream and cut into 12 "cake pieces". Fill the rest of the cream into a piping bag with star-shaped spout and decorate each piece with a cream tuff, blueberries and mint leaves.
Results in about 12 pieces.