Preheat the oven to 350 degrees F (175 degrees C). Butter a 9 inch square baking dish.
In a medium bowl, mix together the sweet potatoes, white sugar, eggs, 2 1/2 tablespoons melted butter, milk, salt and vanilla until well blended. Spread evenly in the prepared baking dish.
In a separate bowl, stir together the light brown sugar, pecans and flour. Stir in remaining 2 1/2 tablespoons of butter to make the crumb topping. Spread topping over the sweet potatoes.
Bake for 25 to 30 minutes in the preheated oven, until topping is browned and crispy.
