In a mixing bowl, combine the mashed potatoes, flour, egg, and salt to form a dough.

Roll out the dough to about 1/4 inch thickness using a rolling pin.

Cut the dough into squares large enough to wrap around a plum.

Place a teaspoon of sugar in the cavity of each plum.

Wrap each plum with a dough square, sealing the edges tightly.

Bring a large pot of salted water to a boil.

Carefully drop the dumplings into the boiling water and cook until they float to the surface, about 10 minutes.

Use a slotted spoon to remove the dumplings and drain on a colander.

In a pan, melt the butter and toast the breadcrumbs until golden brown.

Roll the dumplings in the toasted breadcrumbs to coat them evenly.

Mix cinnamon and powdered sugar, then sprinkle over the dumplings before serving.