Place potatoes in a large pot and cover with cold water. Bring to a boil over high heat. Cook until you and easily pierce the potatoes with a knife.
Drain potatoes and return them to the empty cooking pot. Place on the stove top (not on the hot eye), to let them dry.
Add butter, milk, sour cream, salt and pepper. Mash together with a fork or potato masher until desired consistency. (We like ours a little lumpy.)
Stir in milk, fold in half the cheddar cheese. Add more milk to desired consistency.
Spread the potato mixture into a glass casserole that has been sprayed with non stick spray. Top with remaining cheddar cheese.
Bake uncovered for 20-30 minutes until cheese is bubbly and potatoes are heated through. Broil for 3 or 5 minutes until cheese is golden and slightly brown.