Scrub yams or sweet potatoes. Cut off woody portions and ends.
In a 4-quart Dutch oven cook yams, covered, in enough boiling, salted water to cover for 25 to 30 minutes or until tender. Drain and cool slightly. Peel and cup up yams.
Mash yams with a potato masher or with an electric mixer on low speed. Add milk, brown sugar, butter or margarine, egg, vanilla, and salt. Beat until fluffy. Add additional milk, if desired. Turn into a greased 10x6x2-inch baking dish.
In a small saucepan melt butter or margarine over low heat. Stir in brown sugar, flour, and pecans. Mix well. Spoon pecan mixture over casserole.
Bake in a 350 degree F oven for 30 to 35 minutes or until heated through. Serves 8.