Sieve the flour and baking powder onto the work surface. Press a depression in the middle. Add salt, fat and sour cream, mix with some flour and knead to a smooth dough.
Put the dough in a cool place for about 30 minutes. In the meantime, cook the rice in boiling salted water for about 15 minutes. Boil 3 eggs for 10 minutes, rinse cold, peel and chop finely. Wash, clean and finely chop the mushrooms.
Heat the oil in a pan and fry the mushrooms for about 5 minutes while turning. Wash parsley, dab dry and chop finely. Drain rice, rinse cold and let drain. Mix rice, mushrooms, eggs, mustard, sour cream and parsley, season with salt and pepper.
Roll out the dough on a floured work surface (40x50 cm). Cut into squares (10x10 cm). Separate remaining egg. Spread the filling with a teaspoon over the squares, brush the edges with egg white and fold half of it over to form a triangle.
Press the edges together and press them together again with a fork. Beat the egg yolks with a fork and brush the surfaces of the dumplings with it. Place on a baking tray lined with baking paper and bake in a preheated oven (electric range: 200 °C / gas: level 3) for approx. 25 minutes.
Arrange on a board and garnish as desired with salad and parsley.