Spray a 9x13-inch baking dish with nonstick cooking spray and set aside.
In a large bowl, stir together 3 cups graham cracker crumbs and ½ cup (1 stick) salted butter, melted until combined well.
Press the crust into the bottom of the baking dish and place in to the freezer while you make the filling.
In a large bowl, whip 16 ounces (2 blocks) cream cheese, softened to room temperature with 1 cup powdered sugar and 1 teaspoon vanilla extract until smooth.
In another large bowl, whisk 2 (3.4 ounce) boxes instant pistachio pudding mix with 3 ½ cups cold milk until thickened.
Smooth the cream cheese mixture over the graham cracker crumbs in an even layer.
Spread the pistachio pudding over the cream cheese in an even layer.
Spread 8 ounce tub whipped topping, thawed over the pudding layer.
Cover and place in the refrigerator for at least 4 hours to set or overnight.
Right before serving, add chopped pecans, sweetened coconut flakes and mini marshmallows (if using).
Slice and serve.
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.