In a large bowl, stir together the flour and salt. In another bowl, whisk together the butter, sour cream, eggs, egg yolk, and oil. Stir the wet ingredients into the flour mixture until well blended. Cover the bowl with a towel and let sit for 15 to 20 minutes.
Divide the dough into 2 pieces. Roll out dough very thinly adding a little flour to keep dough from sticking. Cut into 3-inch circles with a cookie cutter. Spoon a bit of filling of your choice onto each circle. Lightly moisten the edges with water, then fold over and crimp the edges with a fork.
Drop the pierogi into boiling water until they float, then drain. Do only a few at a time. Then cool slightly.
Melt some butter in a non-stick frying pan and cook pierogi in the butter over medium heat until they become golden brown and slightly crispy. Serve immediately.