STEP 1: Cook and mash potatoes.You can use potato ricer to make them smoother. Let them cool down.
STEP 2: In a big bowl mix together 3/4 of the mashed potatoes, flour, egg and salt.Work it all together till smooth dough.
STEP 3: On floured surface work the potato dough by hands for about 3-4 minutes or until smooth.Form it into a large ball. Le tit rest for about 10 minutes.
STEP 4: Meanwhile in another bowl mix together rest of the mashed potatoes, sour cream and dill.
STEP 5: Roll out the potato dough to little less then 1/4" thickness. Cut it into 3" - 4" squares.
STEP 6: Place tablespoon of potato dill filling in the middle of each square. Fold it over and pinch around with your fingers.
STEP 7: Pour 3/4" of oil into large skillet. Heat it on medium high. Place pierogi into skillet and fry on both sides until golden brown.
STEP 8: Serve with sour cream mixed with fresh chopped dill.