Mix cake as directed on box. Bake in a greased and floured jelly roll pan at 350~ for 15 minutes. Filling: Cook flour and milk until thickened. Cool. Add sugar, margarine and shortening. Beat at high speed for 8-10 minutes. Spread on cake and chill well. Frosting: Melt margarine and cool slightly. Add melted chocolate squares, vanilla, egg, water and confectioners' sugar. Beat by hand. Frost cake.