For the plum cake with crumbles put sugar, flour, salt, egg, butter and in about 110 ml of lukewarm tap water in a baking bowl. Spread the finely crumbled yeast evenly over the top. Knead everything until the dough separates from the edge of the bowl. Cover the dough with a tea towel and let it rise in a warm place for about 60 minutes.
Rinse the plums, cut them in half and remove the seeds. For the crumble, add sugar, flour, cornstarch, vanilla flavoring, cinnamon and margarine to a baking bowl and knead together to make crumbles.
Knead the risen yeast dough briefly. Roll it out on a baking tray lined with baking paper and cover with the plums and sprinkle with the crumble.
Bake in a heated oven at 200 degrees (gas: level 2-3, convection oven: 175 degrees) for about 30 minutes.
Let the plum cake with crumble cool down and serve.