Here's my mother's recipe for them:
1 pound grated potatoes
1 pound flour (adjust this based on the dough)
1 pinch salt
6-12 plums (depends on the size of the plums)
[assume 3 large or 4 small plums per adult]
2 tablespoons ground poppy seeds [I use a mortar & pestel to grind them - and remember to store the seeds in the fridge]
2 tablespoons sugar
3 tablespoons melted butter
Take the boiled potatoes and grate them. [Two notes here, first, don't cook the potatoes in the microwave or you'll get tough skins, which mess up the dough. Second, if you don't want to grate the potatoes on a cheese grater (which is what I used to do) I used a potato masher the last time I made them and it worked. It's not the same, but it's close enough and saves a lot of effort.]
Once the potatoes are well mashed, add the egg, salt and flour. Mix into a dough. Wrap the dough around washed, but not pitted, plums [wet your hands a little when doing this to keep the dough from sticking to them].
Place the plums into boiling water [if it's not boiling you risk the dough falling off], and let boil 10-15 minutes OR UNTIL THEY FLOAT. Mine typically float in about 5 minutes, so check to be sure the dough is fully cooked but be careful not to overcook them or they'll taste unpleasant.
Top with the butter, poppy seeds and sugar.