For the yeast dough mix flour and salt in a mixing bowl. Warm the milk, crumble the yeast into it and dissolve it in it. Add fat, 25 g sugar and the yeast milk to the flour. Mix to a smooth dough with the dough hooks of the hand mixer.
Let the dough rise in a warm place for about 30 minutes. In the meantime clean and wash the Rharbarber and cut it into 3-4 cm long pieces. Drain well. To make the custard, mix the eggs, sour cream, custard powder, vanillin sugar and remaining sugar with the whisk of the hand mixer.
Knead the dough again well and roll out into a circle (approx. 33 cm Ø) on a floured work surface. Pour the dough into a baking tin lined with baking paper with a twist-off base (28.5 cm Ø) and leave to rise in a warm place for another 15 minutes.
Spread the Rharbarber on the dough and pour over the sand casting. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for about 1 hour. If necessary, cover the cake halfway through the baking time.
Remove the cake from the oven and dust the edge with icing sugar. Decorate as desired with lemon balm. Serve with whipped cream.