For the topping, mix together sugar, egg yolk, vanilla sugar and quark. Fold in the egg whites. Pour the mixture into a piping bag.
For the dough, stir together the sugar, egg yolk, vanilla sugar, salt and grated lemon zest until frothy. Gradually stir in the oil and water. Finally fold in the flour and egg whites. Spread the dough on a greased and floured tray.
Sprinkle the quark mixture onto the dough in a grid shape. Bake at 170 ° C (electric stove, top / bottom heat) for about 30 minutes. When cooled, sprinkle with powdered sugar.