You can do the filling the day before! Boil the potatoes and cabbage together, when the potatoes are fork-tender, drain. Place in a medium bowl. Add butter and mash. Stir in corned beef. Refrigerate until ready to use. If making the same day as you want to make the pierogi, allow to cool to room temperature.

In a large bowl, mix together all of the dough ingredients until round ball forms.

If the dough is overly sticky, add a little bit for flour, if crumbly, add a tiny bit more water. Roll out on a clean, floured surface. Roll about to about 1/8 thick.

Use a large (about 3 inch) round biscuit or cookie cutter to cut out circles. Place 1 1/2 teaspoon of filling on one side of the round of dough leaving a 1/4 inch rim around the bottom. Fold the other side and pinch tightly shut. Repeat until all of the rounds are filled. Bring a large pot of water to boil. Add the pierogi and boil until they float to the surface. Remove and serve or saute briefly in butter with some onions, just to brown them, before serving.