Preheat oven to 325°F.
Scoop cooked potatoes into a bowl and mash. It should be about 3 cups. Add white sugar, eggs, salt, butter, milk, and vanilla. Mix together until smooth. Pour into a 9x13 or 11x8 baking dish if you'd like a little thicker.
In separate bowl, mix together the brown sugar, flour, cinnamon and butter with a fork or pastry cutter. You may need to use your hands to create the crumble. Then stir in the pecans. Sprinkle the topping over the sweet potatoes.
Bake sweet potato casserole for 25-30 minutes, or until the topping is lightly browned and caramelized.
Optional: add marshmallows, sprinkle over the top and broil for 2 minutes, or until golden.