Pre-heat oven to 375 degrees F. Spray a 9x13 baking dish with cooking spray; set aside.
Place sweet potatoes in a large soup pot and cover with cold water. Bring to a boil. Simmer sweet potatoes for 15 minutes; or until very tender. Drain and place in a large bowl, let cool slightly; about 5 minutes.
Add sugar, butter, salt and vanilla extract. Mash sweet potatoes with a potato masher. Fold in 1/4 cups pecans.
Put sweet potatoes in baking dish; in a even layer. Sprinkle remaining 1/4 cup pecans on top of sweet potato; then top with marshmallows.
Bake for 20-25 minutes; or until golden.