Cook sweet potatoes in boiling water to cover 30 minutes or until tender. Drain. Let cool to touch; peel and mash potatoes.
Combine mashed sweet potato, brown sugar, and next 5 ingredients in a large mixing bowl; beat at medium speed of an electric mixer until smooth. Spoon mixture into a lightly greased 1 1/2-quart casserole.
Bake at 350° for 30 minutes or until thoroughly heated. Sprinkle with marshmallows and bake 5 additional minutes or until marshmallows are golden.