Preheat oven to 350 degrees F (175 degrees C). Butter one 2 quart baking dish.
Heat, drain and mash sweet potatoes. Combine with them with the white sugar, eggs, 1/3 cup of the melted butter, milk and vanilla. Place in the prepared baking dish.
In a separate bowl combine the brown sugar, chopped pecans, flour and 1/3 cup of the melted butter. Sprinkle over the top of the sweet potato mixture. Bake at 350 degrees F (175 degrees C) for 35 minutes or until a knife inserted near the center comes out clean.