Preheat the oven to 190°C top/bottom heat.
For the shortcrust pastry, mix the wheat flour, almonds, and birch sugar in a mixing bowl.
If you like the recipe sweet, you can use 100g of sugar.
Add the cinnamon, the margarine in flakes and the oat drink and mix with a fork or a mixer to form an even dough.
Line a 20x20cm baking tin with baking paper and place the shortcrust pastry in it. Flatten at the bottom of the mold.
For the filling, mix the yogurt with cornstarch, vanilla, and sugar until it becomes smooth. Spread the mixture evenly on the shortcrust pastry to the edge.
Stone and halve the plums, then toss in a tablespoon of cornstarch with a pinch of cinnamon until the plums are completely coated.
Stick the plums one by one into the yogurt layer.
Mix the flour, sugar, and margarine in a bowl with a fork into crumbles and spread over the cake with the flaked almonds.
Bake for 45 minutes until it gets golden, then serve lukewarm with cream.