1. Preheat oven to 400°. Bake sweet potatoes at 400° for 1 hour or until tender. Let stand until cool to touch (about 20 minutes); peel and mash sweet potatoes. Reduce oven temperature to 350°.
2. Beat mashed sweet potatoes, granulated 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.
3. Combine cornflakes cereal and next 3 ingredients in a small bowl. Sprinkle over casserole in diagonal rows 2 inches apart.
4. Bake at 350° for 30 minutes. Remove from oven; let stand 10 minutes. Sprinkle marshmallows in alternate rows between cornflake mixture; bake 10 minutes. Let stand 10 minutes before serving.