Mix together sweet potatoes, milk, 1/2 cup butter, sugar, eggs and vanilla. Pour into a medium-size greased casserole.
Bake in 350°F oven for 30 minutes, cover with foil till done or bubbling.
Topping: Combine melted butter, flour, brown sugar and pecans (mix just till the mixture is crumble - if you over mix and you'll have the mixture ball up). Sprinkle on hot sweet potatoes mixture and do not cover. Return to oven (reset at 250°F) until mixture has melted. When melted, remove from oven and let it set for 10 or 15 minutes uncovered.