Sweet Potato Casserole

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish. In a large bowl, mix together mashed sweet potatoes, 1/2 cup butter, milk, sugar, vanilla extract, and eggs. Spread sweet potato mixture into the prepared baking dish. In a small bowl, mix together brown sugar and flour. Cut in 1/3 cup butter until mixture is crumbly, then stir in pecans. Sprinkle pecan mixture over the sweet potatoes. Bake for 25 minutes in the preheated oven, or until golden brown. If desired substitute pecan topping for a marshmallow topping. Add the marshmallows to the sweet potato mixture during the last 5 minutes of baking.