Wash plums thoroughly, drain in a sieve, cut in half and stone. For the crumbles, mix flour, 300 g sugar, 1 sachet vanillin sugar, salt, baking powder, almonds and cinnamon in a large mixing bowl. Add egg yolk and fat in flakes, knead with the dough hooks of the hand mixer. Then work into crumbles with floured hands. Grease a fat pan of the oven (32x38 cm) and sprinkle with flour. Put approx. 3/4 of the crumbles into the fat pan and press them evenly with your hands to a base. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 15 minutes.
Meanwhile, for the icing, place the crème fraîche, eggs, 1 packet of vanillin sugar, 125 g sugar and custard powder in a mixing bowl and mix well with the whisks of the hand mixer. Take the base out of the oven, cover evenly with the plums and pour the icing over it. Sprinkle with the rest of the crumbles. Bake at the same temperature for another 45 minutes. Then remove from the oven and allow to cool on a rack. Cut into pieces