Mix the pudding and milk for two minutes with a wire whisk in a large bowl. The pudding mix should dissolve completely.
Mix in the sour cream and whisk until blended completely.
Fold in the whipped topping until it blends completely. A rubber spatula works best for this
In a 9 x 13-inch baking pan, line the bottom with a layer of graham crackers. This may require breaking the ones on the end to line the pan precisely.
Using the rubber spatula, spread the pudding and sour cream mixture onto the graham crackers. Use about half of the mixture. While an even spread is important, you also must be careful to not dislodge any of the crackers on the pan’s bottom.
Create a second graham cracker cake layer by laying the crackers on top of the pudding mixture.
Spread the other half of the pudding and sour cream mixture on top. Once you have it spread evenly using the spatula, you start on the next layer.
Add another graham cracker layer on top of the pudding.
Top the uppermost layer of graham crackers with the whipped chocolate frosting using the spatula. This may take some time since you need to ice the layer without moving any of the crackers.
Use a cake pan cover or wrap plastic wrap or aluminum foil over the top of the pan. A pan with a cover is the preferred choice since you do not want the cover to touch the icing.
Refrigerate the cake at least four hours. If you can let it sit overnight, do so.
Once set, use a spatula to cut the cake for serving.