Set a large pot of salted water on to boil. Cook peeled potatoes in salted water until tender. Drain well and mash thoroughly until smooth and lump-free. Wait until it cools off. Add eggs, salt (if still needed) and gradually flower. Knead on flowered surface until well mixed and firm.
In meantime cut each plum following the vertical line on their skin to remove stone, but try to keep it in one piece. Leave aside.
Roll the dough into a log (about 3cm), then slice into knedle (3- 4 cm). Take a piece of potato dough in your hand, flatten it and place plum in the middle. Sprinkle with sugar and wrap it, creating ball from the dough. Glue knedle very carefully. It is important, since they have tendency of opening during cooking. Leave them on flowered surface. Once all of them are ready, place them in pot of boiling water. Add a spoon of salt and the spoonful of oil on the cooker. Thanks to oil knedle won't get glued to each other. Cook for about 4 minutes, remove from the pot with slotted spoon and serve immediately topped with sweetened soured cream or melted butter with sugar and cinnamon.