Melt butter in a pot. Mix flour, sugar, vanillin sugar and salt in a bowl. Add hot butter and knead into crumbles with the dough hooks of the hand mixer. Grease a baking tray (37 x 32 cm) and spread a good half of the crumble dough as an even base on it and press it down. Add 1 tablespoon of flour to the rest of the crumble dough, toss with the crumbles and allow to cool.
In the meantime spread the plum puree on the crumble base. Bake in the preheated oven, mid-height, (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 25-30 minutes. Let the cake cool down on the tray. Then cut the cake into large triangles. Can be stored well in tins or wrapped in aluminium foil