Mix the butter, sugar, vanilla sugar, salt and egg yolks until frothy. Mix the flour and baking powder, sift and gradually stir in. Add milk while stirring. Spread the dough flat on a baking sheet lined with baking paper.
Clean the rhubarb and cut into pieces about 1 cm in size.
For the meringue, beat the egg whites with sugar until stiff. Use a spatula or wooden spoon to fold the rhubarb pieces and chopped nuts loosely into the meringue. Spread the mixture on the dough.
Bake in the hot oven for 35 minutes.
If you like, dust the cooled cake with powdered sugar before serving.
Tip: If you prefer the rhubarb softer, you can pre-cook the rhubarb pieces in a sieve over steam for 10 minutes (be careful, you`ll quickly turn into a compote!).