Wash the potatoes thoroughly, then place in a large pot and pour in water. Salt the water then cook the potatoes with skin on about 20-30 minutes, until it softens. Check with a fork when you can stick it right through the potatoes. Drain and set aside.
Wash and pit the plums. You can half them or leave them whole (without the pit).
Peel the potatoes while still warm and mash them. Add flour, salt and egg. Knead together by hand into a smooth dough and use it immediately.
Roll the dough out on a floured surface about 0,5 cm thick. Cut the dough into squares about 7 cm in size. You can also cut in circles. At the center place a pitless plum, dust with a teaspoon of sugar and cinnamon, add a smallish dice of butter, then fold the dough over the fruit. Shape into dumplings and make sure to press the dough real good together at the bottom.
Put a pot of salted water to boil. When boiling, gently place dumplings into the water and cook about 10-15 minutes, or until the dumplings float to the top of the water. Once cooked, remove from the pot gently and drain.
In a pan, melt 50 g of butter, then add breadcrumbs and gently fry them while stirring for a couple of minutes.
Take your dumplings, gently place them in the breadcrumbs and butter and roll them around. Then remove from pan, place on a dish and dust with sugar.