Melt the butter and let it cool down a little. Wash, halve and stone the plums. Mix flour, sugar, vanillin sugar, cinnamon, baking powder and salt in a bowl. Add egg and butter.
First knead into crumbles with the dough hooks of the hand mixer, then with your hands. Grease a springform pan (26 cm Ø). Put approx. 2/3 of the crumbles into the form, press to a flat bottom. Spread the plums on the bottom, add the remaining crumbles.
Bake in the preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) on the lower rack for 40-50 minutes. Cover at the end of the baking time if necessary. Let the cake cool down on a cake rack in the mould.
Remove from the mould.