Boil peeled potatoes in lightly salted water over high heat for 15-20 minutes until tender. Drain well. Mash with a potato masher until completely smooth with no lumps. Cool to room temperature. Add flour, potato flour, and egg yolks. Melt 50g of the butter and mix in. Combine everything into a smooth dough.
Heat the rapeseed oil in a pan over medium-high heat. Using a tablespoon, scoop and slide portions of dough into the hot oil to form dumplings. Fry for 2-3 minutes until golden brown, then flip and fry the other side for 2-3 minutes until golden brown. Remove to a plate.
Add the remaining butter to the pan over medium heat. Mix the poppy seeds with the powdered sugar. Add this mixture to the butter and stir for 1-2 minutes until lightly caramelized. Return the dumplings to the pan, turn once to coat with the poppy seed butter, and cook for about 1 minute.
Heat the plums in a separate pan or vessel for 2-3 minutes until warmed through. Arrange the cooked dumplings with the warm plums on a serving plate. Pour the poppy seed butter sauce from the pan over the dumplings and plums. Sprinkle additional powdered sugar on top. Serve warm, or cool and serve as a cold dessert if desired.