Dice marzipan for the crumbles. Put flour, egg, salt, vanilla sugar, sugar, butter in flakes and marzipan in a mixing bowl. First use the dough hooks of the hand mixer, then work into crumbles with your hands.
Wash the plums, drain well in a sieve and stone them. Line a round pizza tray (32 cm Ø) with baking paper. Spread just under 2/3 of the crumbled dough on the base, press down evenly and firmly.
Sprinkle with breadcrumbs. Spread plums on top. Sprinkle with remaining crumbles and brown sugar. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) on the bottom shelf for about 50 minutes.
If necessary, cover the cake after about 35 minutes. Take it out of the oven, place it on a rack and let it cool down. Serve with whipped cream sprinkled with brown sugar.