Preheat your oven to 350°F for 10 minutes. While it heats, grease a 9x13-inch baking dish with cooking spray or butter—coat the bottom and sides evenly so the cake releases easily. Checkpoint: Oven should display 350°F, and your baking dish should be shiny and well-greased with no dry spots.

Pour the entire 20-ounce can of crushed pineapple directly into the bottom of your prepared baking dish—do not drain it, include all the juice. Use a spatula to spread it into an even layer across the entire bottom. Checkpoint: The pineapple should cover the entire bottom of the dish in a uniform, slightly juicy layer about half an inch thick.

Open your box of yellow cake mix and sprinkle the entire dry mixture directly over the pineapple layer. Spread it gently with your fingers or a fork so it covers all the pineapple evenly—do not stir or mix it into the pineapple. It should sit on top like a blanket. Checkpoint: The cake mix should cover the pineapple completely, and you should see no pineapple showing through.

Slice 1/2 cup (1 stick) of cold butter into approximately 12 even pats. Scatter these pats across the entire surface of the cake mix layer, spacing them roughly 2 inches apart. As they bake, they will melt and create pockets of richness throughout. Checkpoint: All visible cake mix should have at least one butter pat nearby; no large dry areas should remain.

In a small mixing bowl, combine 1/2 cup chopped pecans, 1/4 cup brown sugar, 1/4 teaspoon salt, and 1/2 teaspoon vanilla extract. Stir with a fork for 30 seconds until the pecans are lightly coated with brown sugar and the mixture resembles wet sand. Checkpoint: The pecans should smell aromatic, appear glistening, and hold together slightly when squeezed (but not wet).

Sprinkle the pecan mixture evenly over the butter-topped cake layer, distributing it from edge to edge. Some will stick to the butter; some will nestle into gaps—this is perfect and intentional. Checkpoint: You should see a scattered, golden-brown pecan and brown sugar topping across the entire surface with no large bare spots.

Place the baking dish into the preheated 350°F oven on the middle rack. Bake uncovered for 45–50 minutes. Around 30 minutes in, check the top—it should be turning light golden. By 45 minutes, it should be a deeper golden brown with a slight crust forming on the edges. The cake layer beneath should feel set when you gently press the top (it will spring back), and you may see small bubbles around the edges where the pineapple juice is gently boiling. Checkpoint: The top should be golden brown to deep tan, the edges slightly caramelized, and the cake should feel firm when lightly pressed.

Remove the baking dish from the oven using oven mitts—the dish and its contents will be very hot. Place it on a heat-safe trivet or countertop. Let it rest for 10 minutes without moving it. This resting time allows the pineapple layer to firm up slightly and makes serving much cleaner and easier. Checkpoint: After 10 minutes, the surface should look set (no rippling), and a spoon should be able to cut through cleanly into separate layers.