Wash the strawberries, drain well and clean. Clean and wash the rhubarb Cut both into cubes and mix with 40 g sugar and 1 packet of vanillin sugar in a pot. Leave to stand for approx. 30 minutes
Melt 80 g butter in a pot. Mix flour, salt, 60 g sugar and 1 packet of vanilla sugar in a bowl. Mix the yeast in a cup with 1 tablespoon of sugar until the yeast becomes liquid. Add buttermilk to the warm butter. Add egg, lemon zest, yeast and the buttermilk mixture to the flour. Knead with the dough hooks of the hand mixer to a smooth yeast dough. Cover and leave to rise in a warm place for about 45 minutes
Bring strawberries and rhubarb to the boil while stirring occasionally and let it boil for about 1 minute. Mix starch and 2-3 tbsp. cold water. Remove the pot from the heat, stir in the starch and place back on the heat. Let simmer for about 1 minute while stirring. Put the compote into a bowl and let it cool down
Briefly knead the yeast dough again and roll out into a rectangle (approx. 28 x 38 cm) on a lightly floured tea towel. Spread with the rhubarb-strawberry compote. Leave a 2-3 cm wide strip on the long sides and a 1 cm wide strip on the short sides. Roll up the dough from one long side using the cloth. Cut the roll into approx. 10 thick slices. Place the cut surfaces facing down in a greased springform pan (approx. 26 cm Ø) sprinkled with breadcrumbs. Cover and leave to rise for about 15 minutes.
Bake in the preheated oven, 2nd shelf from below (electric oven: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes. After about 20 minutes, spread 40 g soft butter in flakes on the cake and sprinkle with about 2 tbsp. sugar. Bake the cake, covering the last 10 minutes if necessary. Let the cake cool down in the tin on a cake rack. Whipped cream tastes good with it
Waiting time approx. 50 minutes