Sweet Potato Casserole

My sister-in-law found this on Allrecipes and made it for Thanksgiving dinner. It was the best sweet potato casserole that I'd ever had. I highly recommend it.
4 c. sweet potato, cubed
1/2 c. white sugar
2 eggs, beaten
1/2 tsp. salt
4 Tbsp. butter, softened
1/2 c. milk
1/2 tsp. vanilla extract

1/2 c. packed brown sugar
1/3 c. all-purpose flour
3 Tbsp. butter, softened
1/2 c. chopped pecans

Preheat oven to 325 degrees. Put sweet potatoes in a medium saucepan with water to cover. Cook over medium high heat until tender; drain and mash. In a large bowl, mix together the sweet potatoes, sugar, eggs, salt, butter, milk, and vanilla. Mix until smooth. Transfer to a 9x13 baking dish. In a medium bowl, mix the sugar and flour. Cut in the butter until the mixture is course. Stir in the pecans. Sprinkle the mixture over the sweet potato mixture. Bake in the preheated oven 30 minutes or until topping is lightly browned.