Prepare the Brownie Layer: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish. Prepare the brownie batter according to the package instructions. Pour the batter into the prepared baking dish and bake for approximately 20 minutes, or until the brownies are almost set but still slightly undercooked.
Prepare the Pecan Pie Topping: While the brownies are baking, whisk together the granulated sugar, corn syrup, eggs, melted butter, and vanilla extract in a medium bowl until well combined. Stir in the pecans until evenly distributed.
Combine and Bake: Once the brownie layer is partially baked, remove it from the oven. Carefully pour the pecan mixture over the brownie layer, spreading it evenly. Return the dish to the oven and bake for an additional 25–35 minutes, or until the pecan topping is set and only slightly jiggles in the center when gently shaken.
Cool and Serve: Remove the brownies from the oven and allow them to cool completely in the baking dish. Once cooled, cut into squares and serve.