Butter 8" square glass baking dish with 2" sides. Boil potatoes until soft about an hour. Drain. Peel and trim any black spots. Combine all remaining ingredients until mixture is smooth. Transfer into prepared baking dish. (Cover and chill). Preheat oven to 350. Bake until center is set and top begins to brown about 50 minutes. Add marshmallows, and bake an additional 5 minutes or until browned.