Pierogies (potato/cheddar filling )

Pierogies Dough
4 cups all purpose flour
1 teaspoon salt
2 teaspoons vegetable oil
1/4 teaspoon baking powder
1 cup warm water
1 egg beaten
1. In a large bowl, mix together the flour, salt, and baking powder. Make a well in the center.
2. In separate bowl mix together the vegetable oil, warm water, and beaten egg. Pour into the well of the dry ingredients. Knead dough for 8-10 minutes.
3. Cover dough and let rest for 2 hours. Roll out to about 1/8 to 1/4 inch thick.
4. Cut dough with round cookie cutter or top of a glass and fill.
Potato/cheddar filling
2-1/2 pounds potato (peeled and cubed)
½ pound shredded cheddar cheese
salt and pepper to taste
1) Boil potatoes until soft.
2) Whip potatoes and cheddar cheese with mixer, season with salt and pepper to taste).
In center of cut out dough, place ½ tablespoon mashed potatoes. Fold dough over and seal with fingers. Boil filled and sealed Pierogies until they float to the top. Bake at 350 degrees in a casserole dish with 6 tablespoons of butter and sliced or diced onion or onion salt 25-30 minutes or until crisp OR pan fry in butter and onions until crisp.