Fold in Cool Whip until well blended.
Layer a 9x13" pan with graham crackers. Layer 1/2 the pudding mixture, another layer of graham crackers, then pudding mixture and top with graham crackers.
To make chocolate sauce, mix sugar and cocoa in a saucepan to remove lumps. Slowly stir in milk and cook over low heat. Keep stirring until mixture boils; boil for 1 minute. Remove from heat and add margarine, salt and vanilla. Stir and pour over graham crackers.
Refrigerate or freeze.