Sweet Potato Casserole

3 cup mashed, cooked, sweet potatoes(about 2 1/4 pounds)
1/3 cup firmly packed brown sugar
1/3 cup skim milk
2 tablespoon reduced calorie stick melted margarine
1 teaspoon vanilla
1/2 teaspoon salt
2 egg whites, lightly beaten
Vegetable cooking spray
1/2 cup firmly packed brown sugar
1/4 cup flour
2 tablespoon chilled reduced-calorie stick margarine
1/3 cup chopped pecans

Combine the first seven ingredients in a bowl and stir well. Spoon sweet potato mixture into an 8 inch square baking dish coated with cooking spray. Combine 1/2 cup brown sugar and flour in a bowl, and cut in 2 tablespoons chilled margarine with a pastry blender or 2 knives until the mixture resembles coarse meal. Stir in the chopped pecans, and sprinkle over the sweet potato mixture. Bake at 350 degrees f for 30 minutes.