Prepare the oven. Preheat your oven to 350°F and generously spray a 9x13-inch casserole dish with nonstick cooking spray.

Make the topping. In a medium-sized mixing bowl (2-3 quarts), whisk together 1 cup dark brown sugar, dark corn syrup, melted butter, eggs, and vanilla extract until thoroughly combined.

Stir in the pecans. Use a rubber spatula to stir in the 3 cups of pecan halves and pour the mixture into the prepared baking dish, spreading it into an even layer.

Make the cake layer. In a large bowl, use a fork or clean hands to crumble the softened butter into the cake mix until it’s the texture of wet sand, and sprinkle the mixture over the pecan filling.

Add the brown sugar. Sprinkle the remaining ½ cup dark brown sugar over the cake mix layer.

Pour hot water over the top. Carefully pour the hot water over the top of the dry cake mix and brown sugar, but don’t mix it together. If you need to tilt the dish a bit to make sure the cake mix is covered, you can.

Sprinkle more pecans. Sprinkle the remaining pecan halves on top.

Bake, cool, garnish, and serve. Bake for 35-40 minutes or until golden brown. Because all ovens bake differently, check on the cake at the 30-minute mark. The edges should be set, but the center will still have a slight jiggle to it. As the cake cools, the center will firm up but still be soft, unlike a regular sponge cake. Allow the cake to cool completely before drizzling with caramel sauce and serving.