Preheat oven to 375°.
Place the sweet potatoes in a Dutch oven, and cover with cold water. Bring to a boil. Reduce heat, and simmer for 15 minutes or until very tender. Drain; cool slightly.
Place potatoes in a large bowl. Add sugar and next 3 ingredients (through vanilla). Mash sweet potato mixture with a potato masher. Fold in 1/4 cup pecans. Scrape potato mixture into an even layer in an 11 x 7-inch baking dish coated with cooking spray. Sprinkle with remaining 1/4 cup pecans; top with marshmallows. Bake at 375° for 25 minutes or until golden.