Clean, wash and cut the rhubarb into pieces. Boil up the juice, 1 packet of vanilla sugar and 75 g of sugar. Add the rhubarb and simmer over medium heat for about 2 minutes. In the meantime stir sauce powder and 4 tablespoons of water until smooth.
Carefully lift the rhubarb pieces out of the juice with a ladle. Bring the juice to the boil again, stir in the sauce powder and simmer for about 1 minute, stirring continuously. Remove from the heat, carefully fold in the rhubarb pieces and let them cool down.
For the yeast dough mix flour and yeast in a mixing bowl. Add remaining sugar, salt, remaining vanilla sugar, butter in flakes and egg. Warm the milk in a small pot lukewarm. Add to the flour mixture 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. Then knead again with floured hands. Roll out on a floured work surface (approx. 43x28 cm) and cut out 6-7 circles (approx. 14 cm Ø).
Place circles on two baking trays lined with baking paper and let them rise again in a warm place for about 20 minutes. Whisk egg yolk and cream with a fork. Press a larger depression in the middle of each circle.
Spread rhubarb and pudding evenly in it. Brush the edges of the dough with egg yolk and sprinkle with flaked almonds. Bake the trays one after the other in a preheated oven (electric: 175 °C/ gas: level 2) for 20-25 minutes.
Remove from the oven and serve dusted with pistachios and icing sugar and decorated with mint.