1. Separate the eggs. Beat the egg white with 2 tbsp warm water. Beat the egg yolk, sugar and vanilla sugar until creamy.
2. Add the stiffly beaten egg white to the egg yolk. Then mix the flour, baking powder and cornstarch, sieve onto the egg mixture and carefully fold in.
3. Spread the dough onto a baking tray lined with baking paper, bake at 200°C for approx. 15 minutes and turn out onto a sugared tea towel while still hot, roll up and allow to cool.
4. Now soak the gelatine for the filling. Mix the quark, sugar, vanilla sugar, lemon juice and zest. Then squeeze out the gelatine, dissolve it and stir it drop by drop into the quark mixture, then chill. Whip the cream. Then, as soon as the lemon mixture begins to set, fold in the cream.
5. Unroll the sponge cake, spread with cream, roll up again and chill for approx. 3 hours. Dust the roll with icing sugar