Peel and dice potatoes. Place in a deep pot, cover with water, and add salt. Bring to a boil on high. Lower the temperature and continue boiling for about 15 minutes, or until potatoes are soft when you test them with a fork.

Strain potatoes and mash them. Leave aside for an hour to cool. Meanwhile pit each plum by cutting it along its natural longitude on one side, and taking the pit out. Do not cut in half completely, instead leave plums in tact. Take the butter out of the fridge so it's soft.

Dust the working area with plenty of flour. Add mashed potatoes, egg, butter, more flour and start working the ingredients into dough. How much flour you use will depend on the kind of potatoes you end up with. If the potatoes are dry, use less flour. If the potatoes are wet, use a little more. The dough should be on the softer, more elastic side, but not falling apart. It needs to be just tough enough to be worked into dumplings, and hold a plum inside without separating.

Divide the dough it into two loaves. Work each loaf a few minutes more, and then cut each into one-inch dumpling rounds. Meanwhile, fill up a large pot with a gallon or two of water, leaving ample space on the top. (The water level will rise as you place the dumplings in.) Place the pot on stove on highest, cover, and bring to a boil.

Take a pitted plum, place a teaspoon of sugar into it, and transfer onto a dumpling round. Form everything into a ball making sure that the plum is safely secured inside the dough. Repeat until you are out of rounds or plums. The number will vary, but with this recipe you should end up with around 30 dumplings.

By this time the water should be boiling. Uncover it, lower the temperature to medium, and start placing formed dumplings in. They'll sink to the bottom. They are finished when they rise up, so take them out one by one. It may take up to 15-20 minutes for the dumplings to rise, sometimes longer, just make sure you got your eye on the water. Stir the water occasionally so the dumplings don't clump together, and don't put too many in at any one time. As there are a lot of dumplings, you'll have to boil them in rounds.

Meanwhile, simmer breadcrumbs and oil in a larger pan for 3-4 minutes, and prepare a bowl with sugar, and set aside. After taking each dumpling out of the boiling water, roll it in bread crumbs first until it's covered on all sides. Afterward, transfer the dumpling into the bowl with sugar and repeat.

Consume warm. (Optionally serve with sour cream.)