In a medium mixing bowl, whisk the vanilla pudding mix with 3 cups of cold milk until almost ready, about 1-2 minutes. It should be firm within 5 minutes.
Stir or whisk in thawed Cool Whip.
Line the bottom of a 9x13 dish with a layer of graham crackers. You will probably have to break into pieces to completely cover the bottom.
Add half of the pudding mixture over the graham crackers and spread evenly.
Lay another layer of graham crackers over the top of the pudding mixture.
Spread the remaining pudding mixture over this layer of graham crackers.
Add the final layer of graham crackers on top of the pudding mixture.
Remove the lid and aluminum foil top from the frosting container, microwave the frosting for 20-30 seconds, and stir.
Pour the frosting over the graham crackers and spread evenly.
Cover with plastic wrap and refrigerate overnight before serving. Don't be tempted to eat it the same day! The graham crackers need time to soften.
The next day, cut into squares and enjoy cold!