Preheat oven to 325 degrees. Cook sweet potatoes in water over medium heat until tender, drain and mash. Mix sweet potatoes, white sugar, eggs, salt, 4 Tbsp. butter, milk, and vanilla. Mix until smooth. Transfer to a 9x13 baking pan. Mix brown sugar and flour. Cut in 3 Tbsp. butter until mixture is coarse. Stir in pecans and sprinkle over the sweet potatoes. Bake for 30 minutes until topping is lightly brown.