1. Preheat the oven to 350°F (175°C) and grease a 9×13 inch baking dish.
2. In a mixing bowl, combine mashed sweet potatoes, white sugar, melted butter, milk, eggs, and vanilla extract. Spread the mixture into the prepared baking dish.
3. In another bowl, mix together brown sugar, flour, and butter until crumbly. Stir in chopped pecans and sprinkle over the sweet potato mixture.
4. Bake for 25-30 minutes, or until the topping is golden brown and the casserole is heated through.