For the dough, stir the butter or margarine until foamy and add the vanilla sugar. Alternately stir in sugar, eggs and a mixture of cornstarch, flour and baking powder. Spread the dough on a greased baking sheet.
Bake in a preheated oven at 180 ° C for about 15 minutes.
In the meantime, clean the rhubarb for the topping and cut into small pieces. Mix the fat, sugar, vanilla sugar and eggs until frothy. First mix in the quark, then a mixture of cornstarch and baking powder and finally the rhubarb. Spread the topping on the dough plate and finish baking the cake in about 70-80 minutes at the same heat.