Mashed Potato, Cheese, and Chive Gratin

Mashed Potato, Cheese, and Chive Gratin 3 1⁄2 lbs russet potatoes 8 ounces cream cheese, room temperature 1⁄2 cup sour cream 1⁄2 cup whole milk 1 tablespoon butter, chilled 3⁄4 cup fresh chives, chopped (about 3 bunches) 1 tablespoon butter, chilled, cut into small pieces Butter a 6 to 8-cup ovenproof dish. Cook potatoes in large pot of boiling salted water until tender, about 12 minutes. Drain potatoes; return to same pot. Add cream cheese and mash well. Mix in sour cream and milk, then chives. Season potatoes with salt and pepper. Spoon potatoes into prepared dish; dot with chilled butter. (Can be made 2 hours ahead; let stand at room temperature.). Preheat oven to 375°F Bake potatoes until heated through and beginning to brown on top, about 30 minutes.