Preheat oven to 350°. Place potatoes in a 6-qt. stockpot; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, until tender, 15-20 minutes. Drain; return to pot., Mash potatoes, gradually adding milk, butter, pepper and salt. Spread into a greased 3-quart baking dish; sprinkle with 1 cup cheese and bacon. Dollop with sour cream; sprinkle with chives and remaining cheese., Bake, uncovered, until heated through and cheese is melted, 20-25 minutes.