In a large mixing bowl, whisk together the instant pistachio pudding mix and whole milk for about 2 minutes until the mixture thickens to a creamy consistency.

Gently fold in the thawed whipped topping into the pudding mixture using a spatula until well combined.

Take a 9 x 13-inch baking dish and layer the bottom with whole graham crackers, placing them side by side to cover the entire surface.

Evenly spread half of the prepared pistachio pudding mixture over the graham crackers.

Place another layer of whole graham crackers on top of the pudding mixture.

Spread the remaining pistachio pudding mixture atop the second layer of graham crackers.

Top the pudding with a final layer of graham crackers, then spread the remaining whipped topping gently over the top layer.

Cover the baking dish with plastic wrap and place it in the refrigerator to chill for at least 4 hours, or ideally overnight.

Before serving, garnish the top with chopped pistachios.