Mise en place. Preheat oven to 325°. Grease a 9" by 9" baking dish.
Place cubed sweet potatoes in a medium pan with water to cover. Bring to a boil. Lower heat to a simmer and simmer until potatoes are tender. Drain. Mash.
In a large bowl, whip together sweet potatoes, white sugar, 1/4 C. of brown sugar, eggs, salt, 4 Tbsp. of butter, milk, and vanilla extract. Mix until smooth. Pour into prepared baking dish.
In a medium bowl, mix remaining 1/2 C. of brown sugar and the flour. Cut in 3 Tbsp. butter until mixture is coarse. Stir in chopped pecans. Sprinkle mixture over sweet potatoes.
Bake in oven for 30 minutes or until topping is slightly browned.