Warm up the milk. Put 250 g flour in a bowl, make a depression in the middle, crumble yeast into it. Add 4 tablespoons of warm milk and 1 teaspoon of sugar and mix gently. Melt 50 g fat in remaining milk.
Place 45 g sugar, salt, 1 packet of vanilla sugar, 1 egg and the milk-fat mixture in the bowl and work into a smooth dough with the dough hooks of the hand mixer. Cover and leave to rise in a warm place for about 30 minutes.
In the meantime, clean and wash the rhubarb, cut it into pieces and sprinkle with 100 g sugar. Mix the remaining flour, sugar, egg, vanillin sugar and baking powder. Melt the remaining fat, add it and work it into crumbles with the dough hook of the hand mixer.
Knead the yeast dough well again and roll out on a greased baking tray. Spread rhubarb on top. Sprinkle with crumbles. Bake in the preheated oven (electric: 200°C/ gas: level 3) for about 30 minutes.
Take the cake out of the oven, let it cool down a bit and sprinkle with sugar crystals. Makes 24 pieces.