In a medium mixing bowl, combine 2 (3.4-ounce) boxes instant vanilla pudding mix and 3 cups milk. Whisk until the pudding mixture begins to thicken.
Gently fold in 1 (8-ounce) container Cool Whip until the mixture is smooth and well combined.
Arrange a single layer of graham cracker squares in the bottom of a 9×13-inch baking dish. Break the crackers as needed to cover the entire bottom.
Spread half of the pudding mixture evenly over the graham crackers.
Add another layer of graham cracker squares over the pudding mixture.
Spread the remaining pudding mixture evenly over the second layer of graham crackers.
Top with a final layer of graham cracker squares, covering the pudding completely.
Cover the dish with plastic wrap and refrigerate for 30 minutes to 1 hour to allow the pudding layer to firm up.
Remove the lid and foil from 1 (16-ounce) tub of chocolate frosting. Microwave the frosting for about 15 seconds, just until softened and easy to spread. Stir well.
Spread the softened chocolate frosting evenly over the top layer of graham crackers.
Cover the dish with plastic wrap and refrigerate overnight. Chilling overnight allows the graham crackers to soften and gives the dessert its best texture.
Slice into 9 servings and serve chilled.