Boil the potatoes in their skins for about 30-40 minutes, let them cool, peel, and mash them.
Cut the plums in half with a knife, not wholly, but enough to remove the core from them.
Melt the butter in the pan, then pour the whole amount of breadcrumbs and fry it, constantly stirring, until it becomes copper-brown.
Put the cooled mashed potatoes in a mixing bowl and add the rest of the ingredients: eggs, salt, sugar, butter at room temperature and semolina.
Mix all the ingredients by hand and knead the dough until it hardens and detaches from the bowl’s walls.
Boil the water in a larger pot and add two teaspoons of sea salt.
Stretch the dough on a work surface sprinkled with flour until it is 0.5 cm (0.2-0.3 inches) thick and cut it into approximately equal-sized squares.
In the middle of each square, place a plum in which you have put a teaspoon of cinnamon sugar.
Take a square of plum dough in your hand, close the corners over the fruit and roll it in the palm until you get a spherical shape like a ball.
Boil the dumplings slowly, and after they have risen to the surface, simmer for another two minutes, after which they can be removed.
Once the dumplings are cooked, take them out, drain the water as best as possible, and roll them in the fried breadcrumbs.
Hungarian plum dumplings must be served warm, dusted with sugar and cinnamon.