Preheat oven to 400°F. Bake sweet potatoes at 400°F for about 1 hour or until tender. Let cool to touch; peel and mash sweet potatoes. Reduce oven temperature to 350°F.
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, pecans, brown sugar, and melted butter in a small bowl. Sprinkle over casserole.
Bake at 350°F for 30 minutes. Remove from oven; let stand 10 minutes. Sprinkle with marshmallows, and bake 10 more minutes. Let stand 10 minutes before serving.