Perogies (Varenyky) (Pyrohy)

Prepare the filling. Combine the flour and the salt In a large mixing bowl. Add the egg yolks, mashed potatoes, melted butter and just enough of the water to form a medium soft dough. Turn the dough out onto a lightly floured surface and need until smooth. [Too much needing will make the deal tough] Divide the dough in half and cover with an inverted bowl. Let stand for 10 minutes. Roll the dough until it is quite thin and cut rounds about 2 to 3 inches wide with a large biscuit cutter. Bring a large pot of water to boil and add a little salt. To each round of dough, add a spoonful of filling and fold over to form a half circle. Press the edges together with well floured fingers, making sure the pierogies are well sealed. Transfer them out to a lightly floured surface and cover them with a clean tea towel so that they do not dry out. Drop a few at a time into the boiling water, stirring gently with a wooden spoon to separate them and keep them from sticking to the bottom of the pot. Boil for 3 to 4 minutes, at which point they should be puffed and bobbing on the surface. Transfer with a slotted spoon to a colander, drain thoroughly in place in a warm bowl. Drizzle melted butter and toss gently to keep them from sticking together. Cover and keep warm while you cook the remaining pierogies. Serve with fried onions and sour cream and serve hot.