Pierce potatoes with fork. Cook on high in microwave until tender, turning once, about 8 minutes per side. Cut open; cool. Spoon potato flesh into bowl; mash until smooth. Measure 3 cups mashed potatoes.
Preheat oven to 350 degrees F. Coat 11/7 baking dish with nonstick cooking spray; set aside. ( I typically use the disposable baking dishes instead.)
Combine mashed sweet potatoes, granulated sugar, milk, 1/3 cup butter, eggs, vanilla and salt in medium bowl; stir to mix well. Spoon into prepared dish.
Combine brown sugar and flour in small bowl; stir to mix well. Sprinkle over sweet potato mixture. Sprinkle with nuts; drizzle with remaining 2 tablespoons of butter. Bake 20 minutes.
Remove from Oven and arrange marshmallow. Bake for another 10 minutes or until the marshmallows are golden brown.