For the plum cake, first cream the butter, sugar, vanilla sugar and 2 yolks. Then mix the flour and baking powder and slowly stir into the cream so that it does not dust. Add the milk and stir well. Beat the 2 egg whites until stiff and gently fold into the cream.
Spread the finished dough on a baking tray with baking paper and place the plums on top. Bake in the oven at 190 °C top/bottom heat for 20 minutes. Afterwards, let the plum cake cool down and arrange it in pieces.