Preheat oven to 400°F. Spray a 13” x 9” baking dish with cooking spray.
Bring a large pot of water to a boil. Boil sweet potatoes until slightly underdone, about 15 minutes. Drain and let cool, then peel and cut in half.
In a saucepan, combine butter, Splenda Sweetener, molasses, and 1 cup marshmallows. Season with cinnamon and nutmeg. Cook over medium heat, stirring occasionally, until marshmallows are melted.
Stir potatoes into marshmallow mixture. While stirring, mash about half the potatoes, and break the others into bite-sized chunks. Transfer to prepared dish.
Bake for 25 minutes. Remove from oven, and scatter remaining marshmallows evenly over the top. Return to oven and bake until marshmallows are golden brown.