Bake sweet potatoes at 400° for about 1 hour or until tender. Let cool to touch; peel and mash sweet potatoes.
Beat mashed sweet potatoes, sugar, and next 5 ingredients at medium speed with an electric mixer until smooth. Spoon potato mixture into a greased 11- x 7-inch baking dish.
Combine cornflakes cereal and next 3 ingredients in a small bowl. Sprinkle diagonally over casserole in rows 2 inches apart.
Bake at 350° for 30 minutes. Remove from oven; let stand 10 minutes. Sprinkle alternate rows with marshmallows; bake 10 additional minutes. Let stand 10 minutes before serving.