Bring sweet potatoes and water to cover to a boil, and cook 20 minutes or until tender; drain.
Beat sweet potatoes, sugar, and next 6 ingredients at medium speed with an electric mixer until smooth. Stir in raisins, if desired. Pour mixture into a lightly greased 13- x 9-inch baking dish.
Bake at 350°, uncovered, for 30 minutes or until bubbly; top evenly with marshmallows. Bake, uncovered, for 15 more minutes or until lightly browned.