For the dough, put the flour in a bowl, mix with the yeast, sugar and salt. Warm the milk with the butter. Add to the flour with the egg and knead into a smooth dough with a hand mixer. Let go for 45 minutes.
Boil the pudding with the sugar and milk and let cool a little while stirring constantly. Stir in the sour cream, set aside.
For the crumble, knead the flour, sugar, vanilla sugar with the butter to a crumbly mass, also set aside.
Roll out the dough to the size of a baking sheet and lay it out on a baking sheet. Spread the pudding mixture on the dough. Spread the rhubarb pieces over it and crumble the crumble on top.
Bake the cake in the preheated oven for about 30-35 minutes at 180 ° C. Allow to cool in the mold. Warning, the cake will not set until later!