Heat an oven to oven 200C/400F/Gas 6.
Leaving the skin on, boil the sweet potatoes in a pot for 20 to 25 minutes, or until tender.
Once cooked and soft to the touch, peel and cut into thick slices. Place in an oven-proof dish and put to one side.
To make the candied sauce, place butter, brown sugar, cinnamon, vanilla, and the juice of one orange in a saucepan and bring to a gentle boil.
Pour candied sauce over potatoes and then bake for ten minutes in the preheated oven.
Remove from the oven and place marshmallows over top. Return the dish to the oven, until the marshmallows have just melted.