In a mixing bowl, combine the graham cracker crumbs, melted butter, and granulated sugar. Mix until the crumbs are fully coated and resemble wet sand. Press the mixture firmly into the bottom of a 9x13-inch baking dish to form an even layer. Set aside.
In another bowl, beat the cream cheese with an electric mixer until smooth. Gradually add the powdered sugar, mixing until fully incorporated. Fold in half of the Cool Whip until well combined. Spread this cream cheese mixture over the graham cracker crust, smoothing it out with a spatula.
In a large mixing bowl, whisk together the pistachio pudding mixes and whole milk until it thickens, which should take about 2 minutes. Pour the pudding over the cream cheese layer, spreading it evenly with a spatula.
Top the pistachio pudding layer with the remaining Cool Whip. Spread it out evenly for a smooth finish. If desired, sprinkle chopped pistachios on top for an extra touch of flavor and crunch.
Cover the baking dish with plastic wrap and refrigerate for at least 4 hours, or overnight if possible. This allows the layers to set properly and the flavors to meld beautifully. When ready to serve, cut into squares and enjoy your delicious Pistachio Lush!