1Heat oven to 450°. Grease bottom and sides of square pan, 8x8x2 inches, with shortening. Mash sweet potatoes and brown sugar. Shape 1/3 cup potato mixture around each marshmallow into a ball.
2Brush 1 sweet potato ball at a time with butter; roll in cornflake crumbs to coat. Place in pan. Bake uncovered 8 to 10 minutes or until coating is light brown.