Cream butter and sugar together. Add eggs and stir. Add sweet potatoes and mix well. Stir in milk, vanilla and salt, making sure all ingredients are mixed well. Pour into pie shell and bake 40 minutes @325°. NOTE: For additional flavor you may want to add 1/2 tsp. each of cinnamon and nutmeg. Pour into a buttered pie plate for pie without using a crust. . Bake 40 minutes @375° or until knife inserted in center comes out clean.