In a large mixing bowl, combine the instant vanilla pudding mix and 4 cups of milk. Whisk together until well combined and the mixture thickens slightly.
Fold in the thawed whipped topping and vanilla extract until smooth and fully incorporated.
In the bottom of the 9x13 inch baking dish, layer a single layer of graham crackers evenly.
Spread half of the pudding mixture over the graham crackers, smoothing it out with a rubber spatula.
Add another layer of graham crackers on top of the pudding mixture.
Spread the remaining pudding mixture on top of this layer of graham crackers.
Finish with one last layer of graham crackers, pressing down gently to ensure it adheres.
Take the chocolate frosting and microwave it for about 20-30 seconds until it's slightly melted and pourable. Stir it until smooth.
Pour the chocolate frosting over the top layer of graham crackers, smoothing it out to cover evenly.
Cover the dish with plastic wrap and refrigerate the cake for at least 4 hours, or overnight for the best texture.
Once chilled and set, cut into squares and serve.