Mix all loose ingredients in a bowl. Add oil and then pour in a cup/250ml of water. Start kneading the dough using a stand mixer or an ordinary hand mixer with "dough hook" attachment.
Gradually pour water in, watching if the dough begins to form together. Pour enough of liquid so that it forms properly. I usually use about 450 ml (under 2 cups).
Turn the mixer off and finish kneading by hand. Thoroughly knead until the dough is completely homogeneous, a little moist and elastic. Form a ball shape and then divide into three parts. Leave one on the counter, and wrap the remaining ones tightly with cling film.
Sprinkle a small amount of potato flour on the top of your work surface. Roll out the dough thinly. Use a glass as your cutter to cut a few circles. Place a spoonful of chosen filling in the middle of each and fold in half. Press the edges firmly.
Throw dumplings into a pot with boiling salted water. When they start to float, cook for another minute. Remove them with a slotted spoon, top with metled butter. Serve.