Separate eggs. Beat egg whites and 2 tablespoons of warm water until stiff. Add sugar while stirring. Add lemon zest. Stir in egg yolks one after the other. Mix buckwheat flour and baking powder, sieve over it and fold in carefully.
Line the bottom of a springform pan (24 cm Ø) with baking paper. Fill in the dough. Bake in the preheated oven (electric: 175 °C/ gas: level 2) for about 25 minutes. Let it cool down a bit and turn out of the tin.
Allow to cool down completely. For the filling, soak gelatine in plenty of cold water. Defrost the blueberries. Separate the eggs. Mix quark, egg yolk, lemon juice, sugar and vanillin sugar.
Fold in blueberries. Squeeze the gelatine, dissolve and stir into the quark mixture. Chill until the mixture begins to set. Beat egg white and salt until stiff. Fold into the quark mixture. Cut the sponge cake once.
Cover the base with the edge of the springform pan. Spread half of the curd on top. Place the 2nd sponge cake base on top. Spread the remaining quark on top. Leave to set completely in the refrigerator for about 1 hour.
Serve decorated with mint leaves. Makes about 12 pieces.