Preheat oven to 400 degrees F (200 degrees C). Spray a 9x13 inch baking dish with cooking spray.
Bring a large pot of water to a boil. Boil sweet potatoes until slightly underdone, about 15 minutes. Drain, cool, and peel.
In a large saucepan over medium heat, combine butter, SPLENDA® Granulated Sweetener, molasses, and 1 cup marshmallows. Season with cinnamon and nutmeg. Cook, stirring occasionally, until marshmallows are melted.
Stir potatoes into marshmallow sauce. While stirring, mash about half of the potatoes, and break the others into bite-sized chunks. Transfer to prepared dish.
Bake in preheated oven for 25 minutes. Remove from oven, and cover top evenly with remaining marshmallows. Return to oven, and bake until marshmallows are golden brown.