Preheat your oven to 350°F and grease a 9x13-inch baking dish. In a large bowl, mix the cake mix, pistachio pudding mix, eggs, oil, crushed pineapple (with juice), and milk until smooth.
Pour the batter into the prepared baking dish and spread evenly. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool completely before frosting.
In a bowl, whisk together the pistachio pudding mix and cold milk until thickened. Fold in the whipped topping until fully combined and smooth.
Spread the frosting evenly over the cooled cake. Sprinkle chopped pistachios on top for extra crunch and flavor.
Chill the cake for at least 30 minutes before slicing. Serve cold for a refreshing, creamy, and nutty dessert. Pair it with Pineapple Ice Cream for the ultimate tropical treat!