To make ahead, pour mashed potatoes into 1 1/2-quart ungreased casserole dish. Cover; refrigerate. To reheat in oven, place 1 tablespoon butter, cut into small cubes over potatoes, if desired. Cover with aluminum foil; bake in 350°F. oven 30 minutes or until heated through. To reheat in microwave, cover with vented plastic food wrap. Microwave 5 minutes; stir. Continue microwaving 3-4 minutes or until heated through. Top with an additional 1 tablespoon butter, cut into small cubes, if desired.