Heat the oven to 350 °F and coat a 9x13 cake pan with nonstick spray. The pan should be fully greased so the moist cake releases cleanly at serving time.

Place the dry yellow cake mix, instant pistachio pudding mix, vegetable oil, eggs, and the entire can of crushed pineapple with its juice into a large bowl, then beat with a hand mixer or paddle mixer on medium-high for 1 to 2 minutes. The batter will look thick and very lumpy at first because of the pineapple, then turn evenly mixed without needing more than 2 minutes.

Spread the batter into the prepared pan and smooth the top into an even layer. An even surface helps the cake bake level and keeps the frosting from sliding into low spots later.

Bake for 35 to 40 minutes until a toothpick inserted in the center comes out clean. Look for the edges to slightly pull away from the sides of the pan, which is a strong sign the cake is done.

Set the pan on a wire rack and cool the cake completely to room temperature, about 1 hour. The top should feel cool before frosting so the whipped topping stays fluffy instead of melting.

Whisk the half and half milk, heavy cream, and instant pistachio pudding mix together in a large bowl until the mixture is smooth and starts to thicken, then let it stand for 5 minutes. It should shift from pourable to softly mounded, which means the base is ready for the whipped topping.

Fold in the thawed Cool Whip until the frosting is evenly combined, then spread it over the cooled cake. The finished frosting should look fluffy and light, not runny or dense.

Serve the cake right away or cover the pan and refrigerate it for 2 to 6 hours, or up to overnight, before serving, then add chopped pistachios if using. Chilling firms the frosting and makes the slices cleaner and neater.