Mix pudding as directed on box but add only 3 cups of milk instead of 4 cups. Add Cool Whip in the pudding mixture.
Layer graham crackers in the bottom of a 9x13-inch pan. Then add 1/2 of pudding mixture, then layer graham crackers then the rest of pudding mixture. Layer the top with the last of the graham crackers.
Melt chocolate frosting in microwave, then pour over the top of the graham crackers. Then refrigerate overnight.