Preheat oven to 400 degrees. Roast sweet potatoes wrapped in foil for 1 hr or until tender. Scoop out into medium size bowl and mash.
Lower oven to 325 degrees. .
Mix together the sweet potatoes, white sugar, eggs, salt, butter, milk and vanilla extract. Mix until smooth. Transfer to a 9x13 inch baking dish.
In small bowl, mix the brown sugar and flour. Cut in the butter until the mixture is coarse (I do this step in the food processor). Stir in the pecans. Sprinkle the mixture over the sweet potato mixture.
Bake in the preheated oven 30 minutes, or until the topping is lightly brown.