1. Preheat oven to 425°.
2. Place sweet potatoes in a shallow baking pan lined with foil. Bake at 425° for 1 hour or until tender. Reduce oven temperature to 350°.
3. Peel sweet potatoes; place in a large bowl. Add butter and next 3 ingredients (through cinnamon); mash with a potato masher until pureed. Spoon potato mixture into a 2-quart glass or ceramic baking dish coated with cooking spray.
4. Bake at 350° for 10 minutes. Top with marshmallows; bake an additional 10 minutes or until marshmallows are lightly browned.