In a stand mixer with a hook attachment mix all of the ingredients for the dough together until you get a smooth and springy dough, about 4 minutes. Cover and let rest for 1 hour.
To fill dumplings
On a floured surface roll the dough out into a 12-inch square and cut into 16 squares. Working with once piece at a time put a square of dough into the palm of your hand, add your fruit and sugar if desired. Pinch the dough together to fully enclose the fruit.
Place the dumplings seam side down on a flowered surface and cover with a cloth to prevent them drying out while you shape the rest of the dumplings.
Put a large pot of lightly salted water on to boil.
Turn down to a gentle simmer and put in 6 dumplings at a time, cook for 8 minutes, turning occasionally. Remove with a slotted spoon. If you are doing bigger firm fruit, poke it with a knife to check for how soft the fruit is.
Grind the poppy seeds in a food processor or spice mill until they start to clump together, mix in the powdered sugar.
Drizzle the warm dumplings with melted butter and sprinkle on the poopy seed mixture.