Butter 13 x 9 inch pan and cover bottom with layer whole graham crackers. Mix pudding mix and milk beating well. Fold in whipped topping. Spread half of mixture over crackers in pan. Cover with another layer of crackers. Spread remaining mix over crackers. Top with a layer of crackers. In saucepan, combine sugar, salt, cocoa and milk. Bring to a boil and boil one minute. Remove from heat and add butter and vanilla. Beat until smooth and spread on top of crackers. Refrigerate. Much more delicious if not served until next day.