In Poland plum cake is often a yeast cake baked as a traybake cake. This one also has a crumble toppiing. It's a large cake that tastes best the same day so bake it when you need a cake for many people.
In a small bowl stir the yeast with half of the milk and set to one side till it starts to foam, about 10 minutes.
Melt margarine in a small saucepan and set aside to cool.
In a bowl beat eggs with the yolks, sugar and vanilla sugar.
Sift flour into a large bowl, add the yeast mixture, beaten eggs, remaining warm milk and melted margarine. Knead with your hand or an electric mixer till the smooth and elastic. The dough should be slightly sticky. Cover the bowl with clean tea towel and leave to sit at room temperature until it doubles volume.
Make the topping
In a large bowl combine butter, sugar, flour, vanilla sugar and baking powder. Mix with your hands until well combined but still crumbly.
Preheat your oven to 180 C / Gas 4. Line a large baking tray of 44x28cm or a similar size with greaseproof paper.
When yeast dough has risen, dust your hands with flour and transfer the dough to the baking tray. Roll the dough out on the baking tray and let it come up the sides to form an edge.
Arrange the plums on top, cut side up. Scatter crumble on top and bake in the preheated oven until golden brown, about 1 hour.
Dust with icing sugar and serve within a few hours.