Dice plums roughly and soak in cranberry juice for about 30 minutes. Grease a baking tray (approx. 35 x 40 cm) slightly (so that the baking paper does not slip) and then cover with baking paper.
For the crumble dough, knead flour, sugar, baking powder, 1 pinch of salt, egg, butter in pieces and 3-4 tablespoons of ice-cold water in a large mixing bowl using the dough hooks of the mixer. Then work into crumbles with your hands.
Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Put approx. 2/3 of the crumbled dough on the tray and press it down with your hands to a smooth base, if necessary roll out evenly thin with a cake roll.
Finely puree the soaked plums with a hand blender and spread the puree evenly on the crumble base.
Knead oat flakes and cornflakes under the remaining crumbled dough. Spread on the plum puree. Bake in a hot oven on the lowest shelf for 45-50 minutes. Let it cool down. Whipped cream tastes good with it.