Cook the potatoes: Peel the potatoes, chop any larger ones so they’re all a similar size. Add the potatoes into a large pot, pour over cold water, salt the water. Cook the potatoes until fork-tender, don't overcook them. Drain the potatoes, put the pot back on the burner, and heat the potatoes for a minute or two, stirring, until excess moisture has evaporated.
Mash the potatoes while they are still hot. The best is potato ricer but you can also use a potato masher.
Make the dough: Add all the ingredients into a large bowl and stir them together until well combined (with a spoon or your hands, don't mix for too long, but the dough should stick together, don't knead the dough, just stir everything together until combined). If the dough is too dry, try to add some more melted butter, if it's too wet, add a little more potato starch. Try not to add too much starch so that the dumplings are not too dense.
Form the dumplings: Divide your dough into 8 parts and form round balls (you can weigh your dumplings, each should weigh about 3.5 oz (100g)). To keep the dumplings from sticking to your hands, you can wet your hands with water or dust them with flour.
Cook the dumplings: If you're making these dumplings for the first time, I recommend to test cook the first one to make sure it stays together, before you cook the rest. Add some more potato starch if your dough falls apart. Add the dumplings to very hot, but not boiling salted water. They should only simmer. The best way to do that is to bring the water to a boil, then reduce the heat to medium-low and wait until the water stops cooking. It is not enough to pour boiling water into a pot and add the dumplings, you need to wait until the water comes to the boil again and then stops boiling. Don't overcrowd the pot, if you add too many dumplings at once, the water temperature will drop too much and the dumplings will start to fall apart (cook them in batches). The dumplings will sink at the bottom first, so after a couple of minutes stir them with a wooden spoon to make sure they don't stick to the bottom of the pot. It takes about 13 minutes to cook the dumplings - after this time they will float to the surface of the water.
Take them out of the water with a slotted spoon and serve.