After the potatoes are boiled all the way through (you may check using a fork), wait for them to cool down just enough so it is safe to touch them, then begin peeling and crushing/grating them.
In a bowl place the potatoes along with the two eggs, the salt, and start kneading while slowly adding in the flour. Continue kneading until you reach a smooth consistency. The dough shouldn't be too sticky, nor too dry. Adjust the quantity of flour as needed.
Once the dough is ready, roll it into a sheet (roughly 1cm thick) and cut it into circles or triangles that are slightly larger than your plums. Prepare each plum by tearing them in halves and discarding the stone.
Place a plum/half plum (depending on size) in the center of the dough, and add 1 tsp of sugar on top if the plum is not sweet enough (this will also give it a slightly caramelized taste which some people may prefer). Cover the plum/half plum entirely with the dough, then roll into a ball shape on a flat surface.
Place the formed dumplings in a pot full of boiling (and slightly salted) water for 10-15 minutes. It is important not to overcrowd the pot, each dumpling should be free to float at the top of the pot. Do not hesitate to add them in smaller batches if necessary. It may take 2-3 minutes before each dumpling rises to the surface, use a spatula or a wooden spoon to ensure they don't stick to the bottom of the pot.
While the dumplings are boiling prepare the breadcrumb coating: melt the butter in a pan and add the breadcrumbs and sugar, mixing well. Do not let the sugar caramelize too much, it will make the dumplings taste burnt.
Once each dumpling is ready, take it out of the pot (draining any excess liquid) and roll it around in the breadcrumbs. Place in a bowl and wait to cool down. These dumplings are best served warm with some caster sugar sprinkled on top