Warm up about 1/3 of the milk with 1 tablespoon of sugar in a pot lukewarm. Dissolve yeast in it. Put 500 g flour in a bowl and press a depression into it. Add the yeast mixture and cover with flour. Leave in a warm place for about 5 minutes until the flour breaks open.
Dice 100 g butter. Add eggs, butter, remaining milk, 90 g sugar, rum, lemon fruit and vanilla sugar and knead to a smooth dough. Cover and leave to rise in a warm place for about 30 minutes. In the meantime knead remaining butter with 50 g sugar, remaining flour and cinnamon to a crumbly dough. Put in a cool place. Wash and clean the rhubarb. Cut into pieces of about 2 cm. Roll out the dough on a greased and floured fat pan of the oven. Cover and leave to rise in a warm place for about 15 minutes. Spread rhubarb on top and sprinkle with the remaining sugar.
Cut into pieces of about 2 cm. Roll out the dough on a greased and floured fat pan of the oven. Cover and leave to rise in a warm place for about 15 minutes. Spread rhubarb on top and sprinkle with the remaining sugar. Spread crumbles on top and bake in the preheated oven (electric: 175 °C/ gas: level 2) on the 2nd rack from the bottom for 25-30 minutes. Remove from the oven, let cool for 10-15 minutes and serve lukewarm. Serve with whipped cream