Make-Ahead Ranch Mashed Potatoes

Combine potatoes, water and salt in 2-quart saucepan. Cook over high heat until water comes to a boil. Reduce heat to low. Cover; cook until potatoes are fork tender (15 to 20 minutes). Drain. Mash potatoes with potato masher or electric mixer, gradually adding 2/3 cup milk. Stir in sour cream and dressing mix. Spread potatoes evenly into 1 1/2-quart casserole. Pour remaining milk over potatoes. (DO NOT MIX.) Dot with butter. Cover; refrigerate 4 hours or overnight. Heat oven to 400 °F (205°C). Bake uncovered for 20 minutes; stir. Continue baking for 20 to 30 minutes or until heated through.