Prep the Oven & Pan: Preheat your kitchen oven to 350°F (175°C) and lightly grease a standard 9x13-inch deep baking pan with non-stick cooking spray.
Mix the Tropical Base: Open your can of crushed pineapple over a measuring cup, pressing down with a spoon to catch all the liquid. Take that beautiful pineapple juice and use it in place of the water required by your boxed cake mix. If you run a little short of the box's liquid requirement, just add a splash of water to fill the gap. Mix the cake batter together with the oil and eggs as directed on the box.
Bake and Cool: Pour the smooth batter into your prepared pan. Bake according to the box instructions (usually 28-33 minutes) until the top is a light golden brown and a toothpick inserted in the center comes out clean. Remove from the oven and let it sit on the counter to cool down for about 12 minutes.
Poke the Holes: Grab a wooden spoon with a thick, round handle. While the cake is still warm, gently poke deep holes all over the surface of the cake, spacing them out evenly in a grid pattern. Aim to go about three-quarters of the way down into the cake without pushing all the way through the bottom.
Make the Pourable Pudding: In a medium bowl, whisk together both packages of instant pudding mix with the 4 cups of cold milk. Whisk vigorously for about 1 to 2 minutes until it just begins to thicken but is still thin enough to easily pour. Stir the saved drained crushed pineapple directly into the pudding.
The Big Pour: Immediately pour the pudding and pineapple mixture slowly all over the warm cake, using the back of a spoon to gently push the pineapple bits right down into the holes. Wrap the pan tightly in plastic wrap and pop it into the refrigerator for at least 1 hour to let the pudding set and soak into the crumb.
Toast the Coconut Crunch: While the cake chills, spread your shredded coconut flakes in a single layer on a clean baking sheet. Pop them into the 350°F oven for exactly 3 minutes. Stir them around halfway through. Watch them closely—remove them the second they turn an even, warm golden color, then transfer them to a cool plate.
Frost and Decorate: Take the chilled cake out of the fridge. Spread your thawed whipped topping evenly across the surface using a spatula. Go wild and sprinkle the cooled, crunchy toasted coconut all over the top. If you want to make it look extra festive, dot the top with a few bright red maraschino cherries. Slice, serve cold, and enjoy!
