Sieve the flour and baking powder into a bowl. Make a depression in the middle. Add salt and fat and mix with some flour. Then add the egg and the sour cream and knead everything into a smooth dough.
Cover and let it rest in the refrigerator for 45 minutes. For the filling, cook the rice in boiling salted water for 15 minutes. Boil the eggs hard for ten minutes. Quench cold, peel, let cool and cut into fine cubes.
Clean, wash, drain and finely chop the mushrooms. Wash and chop parsley. Mix mushrooms, eggs, rice and parsley and season with salt and pepper. Roll out the dough on a floured work surface to a rectangle (40x50 cm).
Cut into squares (10x10 cm). Put a tablespoon of filling on each square. Brush the edges with egg yolk and close the corners as a triangle. Press the edges together with a fork. Brush the dumplings with egg yolk and bake on a greased baking tray in a preheated oven (electric cooker: 200 °C/ gas: level 3) for 20-25 minutes.
A fresh mixed salad tastes good with it.