Mix 6 tablespoons cocoa, 1/4 cup water, and 5 tablespoons cornstarch until smooth. Add more water if necessary. Place in saucepan and gradually add 1 quart of whole milk, stirring over medium heat. Slowly add 2/3 cup sugar. Stir until pudding bubbles and thickens. Remove from heat and cool. Layer a 9 x 9 inch pan with graham crackers. Spread 1/3 of pudding over it. Repeat ending with pudding. Sprinkle top with crushed graham crackers. Chill until set.