Preheat oven to 350°. Mash potatoes in large mixing bowl. While still warm, fold in sugar, milk, beaten eggs, vanilla and melted butter. Stir with wooden spoon until well combined.
Turn sweet potato mixture into 2-quart casserole dish. In small bowl, combine light brown sugar, flour, melted butter and chopped pecans. Spoon this topping evenly over sweet potato mixture. Bake about 40 minutes or until topping is crisp, pecans have toasted and sweet potato mixture is bubbling.