Prepare cake mix as directed on box. Pour batter into a greased 11x17 inch cake pan and bake 12-15 minutes at 350 degrees. Cool. Combine sugar, margarine, shortening, milk, vanilla, and salt. Beat until fluffy (10-15 minutes). Spread over cooled cake. Melt frosting in microwave for 20 seconds to soften. Spread frosting over white cream layer.