Preheat and Prepare: Preheat your oven to 350°F (175°C) and generously grease a 9×13 inch baking dish to prevent sticking.
Mix Wet Ingredients: In a large mixing bowl, combine 2 cups dark brown sugar, 1 cup dark corn syrup, ½ cup salted butter (melted), 3 large eggs, and 2 tsp vanilla extract. Mix until smooth and well combined.
Add Pecans: Stir in 3 cups pecan halves into the mixture, ensuring they are evenly distributed. Pour this mixture into the prepared baking dish, spreading it out evenly.
Add Cake Mix: Evenly sprinkle the 15.25 oz white cake mix over the pecan mixture. Do not stir—the beauty of a dump cake is the layering of flavors.
Butter and Sugar Drizzle: Melt the remaining ¾ cup of salted butter and drizzle it over the cake mix. Sprinkle ¾ cup dark brown sugar on top for extra sweetness.
Add Hot Water: Carefully and gently pour 1 cup of hot water over the top, making sure not to mix it in. You want the water to seep into the layers below.
Add Remaining Pecans: Finally, sprinkle the remaining 1 cup of pecan halves on top for a lovely crunchy finish.
Bake: Place the baking dish into the preheated oven and bake for approximately 35-40 minutes. The cake should be golden brown and have a slight jiggle in the center, which will firm up as it cools.
Cool and Serve: Once baked, remove the cake from the oven and let it cool a bit. It can be served warm or at room temperature, optionally topped with caramel sauce.