In a large mixing bowl, combine the pistachio instant pudding mix with 2 cups of whole milk. Whisk vigorously for about 2 minutes until the mixture thickens and becomes smooth.

Gently fold 8 ounces of whipped topping (Cool Whip) into the pudding mixture using a spatula.

Select a rectangular dish for assembling your cake. Arrange a single layer of graham crackers at the bottom.

Spoon half of the creamy pudding mixture over the layer of graham crackers, spreading it evenly.

Add another layer of graham crackers on top of the pudding mixture, followed by the remaining pistachio pudding.

Cover the assembled cake tightly with plastic wrap or a lid and refrigerate for at least 4 hours.

Just before serving, remove the cake from the refrigerator and sprinkle the top with chopped pistachios.