Cook potatoes with 1/2 teaspoon salt in boiling water to cover in a large Dutch oven 30 minutes or until tender; drain. Cool potatoes slightly; peel and cut into chunks.
Beat potato, butter, and next 3 ingredients, in batches, at medium speed with an electric mixer until smooth. Spoon into a lightly greased 13- x 9-inch baking dish. (If desired, cover and refrigerate up to 3 days, or freeze up to 1 month. Thaw at least 12 hours in refrigerator.)
Bake at 350° for 30 minutes; top with marshmallows, and bake 15 more minutes or until golden.