1In 4-quart saucepan, place potato pieces and add enough water to cover. Heat to boiling; reduce heat to medium-low. Cook uncovered 15 to 20 minutes or until tender. Drain and return to saucepan.
2Heat potatoes over low heat about 1 minute to dry potatoes, shaking pan often to keep potatoes from sticking and burning. (This will help make mashed potatoes fluffy.)
3Heat oven to 350°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray. Add milk, salt and pepper to potatoes. Mash with potato masher until no lumps remain. Beat in sour cream, chives and 1/2 cup of the cheese. Spoon into baking dish. Sprinkle with remaining 1 cup cheese.
4Bake uncovered 10 to 15 minutes or until thoroughly heated and cheese is melted.