Boil whole potatoes until tender. Drain well and mash while warm. Let them cool slightly so excess moisture evaporates.
Mix potatoes with butter, eggs, sugar, lemon zest, and salt. Add flour gradually and mix gently until a soft dough forms. Dough should be smooth and NOT sticky.
Add in the flour, mix together and knead well, adding a bit of water if necessary.
Roll out the dough on a floured cutting board. Divide the dough into pieces and flatten each into a disc.
Add one plum and 1 tsp of sugar on top per piece, then roll each dumpling, flatten it and wrap it around a pitted plum. Carefully seal and roll into smooth balls.
Bring salted water to a gentle boil. Add dumplings and cook until they float. Continue cooking for 4–5 minutes.
Pour water to a big pot and bring it boiling, add a bit of salt. Put in the dumplings when the water is boiling, and cook until they float to the surface. Once they do, cook for another 5 minutes and remove from water.
Melt butter in a pan, add breadcrumbs, and toast until golden. Stir in cinnamon and sugar.
Serve with melted butter, sweetened cream and sprinkle with cinnamon.