Bake sweet potatoes at 400º for about 1 hour or until tender. Let cool to touch. Peel and mash sweet potatoes.
Beat mashed sweet potatoes, sugar, milk, butter, eggs, extract and salt 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 butter in a small bowl.
Sprinkle diagonally over casserole in rows 2 inches apart.
Bake at 350º for 30 minutes.
Remove from oven and let stand 10 minutes.
Sprinkle alternate rows with marshmallows and bake 10 additional minutes.
Let stand 10 minutes before serving.