Cook 2 medium to large potatoes until done, about 20 minutes. Peel and place in a medium bowl. Add 1/4 teaspoon salt and 1 tablespoon butter and mash (it should yield about 2 cups). Let cool to room temperature.
Add egg and mix until well combined. Sift in one cup of flour and knead until soft dough forms. Add another 1/2 cup of flour if the dough sticks to your hands. Cut dough into 8 equal pieces.
With lightly floured hands, take a piece of dough and pat it flat. Place one plum in the center and bring sides of dough over plum; roll it between the hands until it's sealed tight (moist your hands if necessary).
Fill a large pot with water and bring to a boil. Lower the heat to medium. One at a time put dumplings into boiling water (do in batches if necessary). Wait until dumplings rise to the top, about 5 minutes. Cook for 3 minutes longer.
Meanwhile, in a large pan, heat 1/3 cup vegetable oil over medium heat; add 1/3 cup breadcrumbs. Stir until the breadcrumbs are golden brown in color (be careful not to burn them - mine got too dark quickly here). Using a slotted spoon, remove cooked dumplings and add them to the breadcrumbs. Roll them in the pan to coat completely. Transfer to a plate.
In a small bowl combine 1 cup sugar and 1 teaspoon cinnamon.
Serve dumplings topped with a dollop of sour cream and sprinkled with sugar/cinnamon mixture.
Run the blade of a sharp knife around the middle of potatoes before you cook them. This will allow the skin to easily slide off. Run the potatoes under cold water for a few seconds until cool enough to handle. Pull the skin off with your hands.
Potatoes cooked with the skin on retain little to no moisture and thus are a better option for dumplings.
Don't worry if potatoes don't mash smoothly. Small clumps are OK and won't be noticeable once the dumplings are cooked. However, you can run it to the potato ricer for an extra smooth texture.
You can mix a tablespoon of Cream of Wheat with the flour for the dough.
Knead the dough on a floured surface to prevent sticking.
Remove the plum pit (stone) by slightly squeezing the plum with your fingers on the narrow sides (where the stem is and the opposite side). It should easily open, exposing the pit. This method works best with firmer plums, but not the large round kind you usually find in the States.
If your plums are not sweet enough, sprinkle them with a teaspoon of sugar (optional) before wrapping them with the dough. Alternatively, you can dip a small sugar cube in spiced rum and place inside each plum.
Add 1-2 tablespoons butter to toasting the breadcrumbs for extra flavor.
Panko, Japanese-style coarse bread crumbs is not suitable for this recipe.
Please keep in mind that nutritional information is a rough estimate and can vary significantly based on its products.