Make the filling by combining the milk, margarine, cake flour, sugar and vanilla in a medium saucepan and cook ingredients together over low heat until thickened; cool completely.
Beat egg whites and cream of tartar on highest speed until stiff peaks form.
Gradually add sugar while mixing.
Fold egg whites into cooled filling.
Pour into a baked 9 inch pie crust.
Top with whipped topping (homemade or Cool Whip) and sprinkle with nuts, if desired.
Refrigerate at least 3 hours before serving.