Mix the butter and sugar until creamy, add the egg and stir in well. Mix the flour with cornstarch and baking powder, fold into the dough and stir briefly. Spread the dough in a greased springform pan,
Place the plums in a circle on the dough.
Mix the sugar, vanilla sugar, butter, flour and cinnamon and knead into crumble. Spread evenly on the plums.
Bake in the preheated oven at 175 ° C top / bottom heat (convection: 155 ° C, gas: level 2) on the middle rack for 40 to 45 minutes.
The cake tastes particularly good with whipped cream.
Tip: I baked with convection and always put a fireproof cup filled with water in the stove. Remove the cup 10 minutes before the end of baking so that the crumbles are crispy.