Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan with cooking spray or a thin layer of oil; you can also line the bottom with parchment for easier removal.

In a large mixing bowl, add the yellow cake mix, 1 cup crushed pineapple with its juice, eggs, oil, water, and 1 teaspoon vanilla. Use a hand mixer on medium speed for 2 minutes, scraping down the bowl once. The batter will be slightly thicker than regular cake batter with small pineapple pieces.

Pour the batter into the prepared pan and spread it evenly with a spatula. Bake for 25–30 minutes, or until the top is golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.

Set the cake pan on a cooling rack and let the cake cool completely in the pan, about 45–60 minutes. Do not frost while warm or the topping will melt and slide.

While the cake cools, add the well-drained crushed pineapple, instant vanilla pudding mix, sour cream, and 1 teaspoon vanilla to a large bowl. Stir or whisk until the pudding is dissolved and the mixture is smooth and thick. Let sit for about 5 minutes to thicken slightly.

Gently fold in the thawed whipped topping with a spatula until fully combined. Use light motions so you don’t deflate it. The mixture should be pale yellow, creamy, and fluffy.

Once the cake is completely cool, spread the pineapple whipped topping evenly over the top, all the way to the edges so every slice gets plenty of topping.

Cover the cake tightly with foil or a fitted lid and chill in the refrigerator for at least 2 hours, or up to overnight, to let the flavors meld and the topping set.

Cut the chilled cake into 12–16 squares. If desired, garnish each piece with extra drained pineapple, a maraschino cherry, or a sprinkle of toasted coconut before serving.