Preheat oven to 350°. Place sweet potatoes in a 6-qt. stockpot; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, until tender, 15-20 minutes. Drain potatoes; place in a large bowl., Beat potatoes until smooth. Add the next 9 ingredients; beat until blended., Spread into a greased shallow 2-1/2-qt. baking dish. Bake, uncovered, until heated through, 40-45 minutes. Increase oven setting to 425°., Top casserole with marshmallows. Bake until marshmallows are lightly browned, 3-4 minutes.