Preheat oven to 350℉ and grease a 9x13-inch pan with cooking spray.
In a mixing bowl, stir together the cake mix, 1 box of the pudding mix, eggs, water, and oil until well-combined.
1 box cake mix, 1 3.4-ounce box instant pistachio pudding mix, 3 eggs, 1 1/3 cups water, 1/3 cup oil
Pour the batter in to the prepared pan and bake for 20-22 minutes or until a toothpick pressed into the center comes out clean or with a few moist crumbs. Remove from the oven and let cool for 10 minutes.
Use the handle of a wooden spoon or a large straw to poke holes into the warm cake; set aside.
In a mixing bowl, whisk together the milk and the second box of pudding mix; let set for 5 minutes to thicken.
1 1/2 cups milk, 1 3.4-ounce box instant pistachio pudding mix
Pour the filling over the cake and spread to an even layer.
Refrigerate for 4 hours or overnight.
When ready to serve, spread the whipped topping over the cake and garnish with chopped pistachios, if desired.
1 8-ounce tub whipped topping, chopped pistachios