Cook potatoes with skin on for 30 minutes. Strain them and let them cool down a bit (about 30 minutes). To cool them down faster, splash them with cold water and then let them cool down in ice-cold water.
Once a bit colder, peel potatoes and clean them if there are any black-eyes on the surface. Mash them well. Add flour and salt and knead the dough.
Wash plums. Cut them in half. Remove the pit.
Wet your hands. Take a bit of the dough, enough for one ping-pong ball. Make a ball and flatten it. Place half of a plum in the middle of the dough and then pull the sides of the flattened dough together to close the ball. Roll it around your hands a bit so the cracks fill up and the plum is hidden inside of the ball.
Keep repeating the procedure. If you want, you can add a bit of sugar to the plum before you close the ball. I like these dumplings without sugar inside. Keep wetting your hands, this way the dough won't stick that much.
Once you're done, you can freeze these dumplings. Place them on a large surface so they don't touch each other and freeze them for a few hours. Once frozen, move them to a reusable plastic bag for freezing.
You can also cook them straight away, they are the best that way! Boil-up water, add some salt to it, and cook fresh dumplings for about 15 minutes. After they rise to the surface of the boiling water, they need a few more minutes to really cook well.
Serve them with fried breadcrumbs: warm-up oil so it's reaaaaally hot and add the breadcrumbs. Fry them until they become golden brown or even a bit darker. Then cover the dumplings with them. Sprinkle cinnamon and sugar on top of them - you can even mix sugar and cinnamon together and have a cinnamon mix ready anytime. Now go for it!