In a large bowl, whisk together the pistachio pudding mixes and milk until the mixture is smooth and thick, using either a whisk or an electric mixer at medium speed.

Gently incorporate 1 cup of the whipped topping into the pudding mix until everything is well combined and creamy.

Arrange approximately 6 to 7 graham cracker sheets in one layer on the bottom of a 9x13-inch dish.

Spread half of the pistachio pudding filling evenly over the graham cracker layer.

On top of the pudding layer, add another layer of graham crackers, followed by the remaining pistachio pudding filling.

Finish with a final layer of graham crackers on top.

Cover the top of the cake with the remaining whipped topping and sprinkle on chopped pistachios for added texture if desired.

Refrigerate the cake for at least 4 hours, or ideally overnight, allowing the flavors to meld together.

When ready to serve, slice into 12 to 15 squares and enjoy this delightful treat!