Mix the butter with the egg and sugar until frothy. Sift in the flour and baking powder (this will make the dough even better) and add the vanilla sauce powder mixed with 60 ml milk. Stir again.
Gradually add the rest of the milk while stirring. This will make the dough nice and creamy.
Place the dough on the greased tray or on the baking sheet lined with baking paper and distribute it evenly. Cut the plums in halves and spread them on the dough with the inside facing up, so that everything is covered as much as possible. If the plums are very sour, add sugar to taste.
Bake in the oven preheated to 180 ° C for about 35 minutes on the middle shelf.
Let cool down on a wire rack for about 30 minutes.