It is best to grind the boiled potatoes or put them through a press while still slightly warm. You can also use potatoes left over from dinner, even 2 days ago (kept in the fridge). I prefer them while still warm after cooking because they are softer.
Add the rest of the ingredients to the bowl with the ground potatoes - the egg, both flours and a pinch of salt. Mix all well. There should be no lumps.
Place the dumpling dough on a floured board. Roll them out with your hands into a string about 4 cm thick. Cut neat dumplings with a knife, about 3 cm long. Flatten each dumpling in your hand. Place washed, pitted and slightly dried plums inside - one plum per dough ball. If you have bigger plums, cut them in to halves or quarters. Seal the edges well. Form a neat ball in your hands. Prepare all dumplings in this way. I made 25 pieces.
Bring a pot of water to boil and salt it when it starts to boil.
Place the dumplings in boiling water. Immediately, lightly and carefully stir the noodles. After the dumplings float to the top, wait another 4 minutes. Remove all the dumplings, drain excess water and place them on a large plate.
Finally, melt the butter in a small pan, add the granulated sugar and mix until dissolved. Cover the dumplings with the butter and sugar mixture. Add some sour cream and you're ready to eat.
ENJOY and SMACZNEGO AS ALWAYS!!!