Boil the sweet potatoes until tender. Remove from the water and let cool slightly. Once the potatoes are cool enough remove the skins by simply peeling them off with your fingers.
With an electric mixer or hand masher mix the potatoes with butter, brown sugar, cinnamon and nutmeg until smooth and creamy.
Spread in a baking dish, top with marshmallows and bake at 350F for 20 minutes.