Preheat oven to 220c and oil a 9 inch pan.
Boil potatoes until they're soft.
For the topping, crush cornflakes and combine with chopped pecans, cinnamon, and sugar. Stir in butter until well combined. Set a side.
Mash sweet potatoes with butter and beat with sugar, milk, egg, vanilla, and salt until well combined.
Pour mixture into prepared pan and sprinkle with topping mixture around the outside of the dish leaving room in the centre.
Bake for 20 minutes or until topping is golden.
Take out, add marshmallows to the centre and bake for 5 more minutes.