Mix the butter with 150 g sugar and vanilla sugar until creamy. Add the egg yolks one at a time and stir in. Add flour, baking powder and milk and stir. Put the dough on a greased baking sheet.
Clean the rhubarb and cut into pieces. Beat the egg white until stiff, slowly pouring in 200 g of sugar. Carefully fold the rhubarb under the egg whites. Distribute this mixture evenly on the dough.
Bake in a preheated oven at 180 ° C for approx. 45 minutes.