Boil the potatoes in salted water until tender. Drain, peel, and mash until smooth. Let cool slightly.
Add the flour, egg, butter, and salt. Mix until a soft dough forms.
Wash and halve the plums, removing the pits. Place a small sugar cube and a pinch of cinnamon inside each plum.
Roll the dough out on a lightly floured surface to about ¼ inch (6 mm) thick.
Cut the dough into squares large enough to wrap around each plum.
Place a prepared plum in the center of each square. Fold the dough around the plum and seal completely. Roll gently between your hands to form a smooth dumpling.
Bring a large pot of salted water to a gentle boil. Carefully add the dumplings.
Cook until they float to the surface, then continue cooking for 3–5 minutes.
Meanwhile, melt the butter in a skillet. Add the breadcrumbs and toast over medium heat until golden and fragrant.
Stir in the sugar and cinnamon.
Remove the cooked dumplings with a slotted spoon and drain well.
Roll the warm dumplings in the toasted breadcrumbs until completely coated.
Serve warm with powdered sugar and extra cinnamon if desired.
