Preheat oven to 350°F. Bake potatoes 1 hour or until tender. Let stand 5 min. Cut potatoes in half lengthwise; scoop out centers, leaving 1/8-inch-thick shells.Mash potatoes in medium bowl. Add cheese, milk, butter, salt and pepper; beat with electric mixer on medium speed until fluffy. (If potatoes are dry, beat in small amount of additional milk.) Stir in chives and bacon; spoon evenly into shells. Place in shallow baking dish.Bake 30 min. or until heated through. Garnish with additional chopped chives, if desired.