Preheat oven to 350 degrees. Mash potatoes in a large mixing bowl. While still warm, fold in sugar, milk, beaten eggs, vanilla and 1/2 cup melted butter. Stir with wooden spoon until well combined. Put mixture in a 2-quart casserole dish.
In a small bowl, combine light brown sugar, flour, 1/3 cup 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.