Boil the potatoes whole until they are tender when pierced with the tip of a knife. Drain the potatoes and peel them while they are still hot.
Mash the potatoes with the butter and salt. Add the flour (or gluten-free flour + psyllium husk powder) and mix thoroughly.
Mix in the egg and knead for 3 to 4 minutes to form a soft, elastic dough.
The dough should still be warm as you roll it out on a floured work surface to a thickness of about ⅓-inch/8.5mm (a rectangle of roughly 10x12.5 inches/25.5x32cm).
Cut the dough into twenty 2½-inch squares (6.5cm).
Place a whole plum in the center of each square. Bring up the corners of the dough over the plum so the edges butt up against each other, and pinch the edges together to join them. (If they don't stick well, add a dab of water to get the edges to stick together.) Gently roll the dumpling into a smooth ball between your palms. Make sure the dough is sealed all the way around the plum, with no gaps or thin spots - pinch or press it back together if it is.You can prepare and form all the dumplings up to this point and let them sit on a floured plate or pan, not touching each other (cover the whole plate in plastic food wrap) for a few hours or up to a whole day ahead (refrigerate them if it's more than a few hours).
Fill a large pot with about 4 inches (10cm) of water and bring it to a boil. Drop in the dumplings, as many as will fit in a single layer, not crowded. Lower the heat slightly - the water should not come back to a boil, but stay just below (if it boils vigorously, the dumplings could fall apart). Cook until they rise to the top (about 8-10 minutes). Remove the dumplings with a slotted spoon and set them onto a plate or pan to drain. Cook the rest of the dumplings in batches, keeping the finished ones warm in a low oven.
In a skillet, melt the butter and oil over medium heat. Fry the breadcrumbs until they are golden. Put a few dumplings at a time into the breadcrumbs and gently shake the skillet back and forth until the dumplings are completely covered in the buttered crumbs.You can keep the crumb-coated dumplings warm in a 175°F (80°C) oven - no longer than 20 minutes (or the crumbs will lose their crispness).
Stir together the sugar and cinnamon and serve with the plum dumplings.
To eat, cut open each dumpling, remove the prune pit, and sprinkle liberally with the cinnamon sugar.
Makes 20 plum dumplings. If serving these for a meal, four or five dumplings per person would probably be enough. If serving them as a dessert or snack, one or two dumplings per person would be a good portion.
Notes
Fruit Substitutions: Instead of prune plums, you can use Damson plums, small apricots (to make Marillenknödel), large chunks of peeled and cored apples, chunks of banana, or a small handful of blueberries in each dumpling.Plum Dumplings can be frozen. Lay the formed, uncooked dumplings in a single layer without touching on a cookie sheet and freeze until solid. Then put them into a freezer bag or container and freeze them for up to 6 months. Cook them in boiling water from frozen - they will take 5 minutes longer than when not frozen.*You can serve the plum dumplings without the breadcrumb coating - just cover them liberally in melted butter and sprinkle them with cinnamon sugar. Or you can also serve them with vanilla sauce, lightly sweetened whipped cream, or vanilla ice cream (with or without the breadcrumb coating).*Use any leftover cinnamon sugar in other baking, or sprinkle it on hot buttered toast (to make cinnamon toast).