Preheat the Oven: Position a rack in the middle of the oven and preheat it to 325°F. Generously coat a 9-inch square baking pan with cooking spray. Line the bottom and sides with two overlapping pieces of parchment paper, arranging them so the paper extends over all four edges by a few inches.

Prepare the Brownie Batter: Follow the package instructions to prepare the brownie batter. Transfer the batter to the prepared baking pan and smooth the top. Bake until the top is dry and smooth (it won’t look crinkly) and a tester inserted into the center comes out clean, about 30 minutes.

Make the Pecan Pie Topping: While the brownies bake, prepare the pecan topping. In a medium saucepan, combine the butter, light brown sugar, corn syrup, heavy cream, vanilla extract, and kosher salt. Bring the mixture to a boil over medium heat, stirring occasionally. Once it boils, reduce the heat and simmer, stirring constantly, for 2 to 3 minutes. Remove from the heat and stir in the chopped pecans until evenly coated.

Add the Pecan Topping: When the brownies are done baking, remove them from the oven and increase the oven temperature to 350°F. Evenly spoon the pecan pie filling over the brownies, spreading it out gently. Return the pan to the oven and bake until the pecan topping is bubbling all over, particularly in the center, about 25 to 30 minutes.

Cool and Serve: Remove the pan from the oven and place it on a wire rack to cool to room temperature, about 1 1/2 hours. Once cooled, use the parchment paper to lift the brownie slab out of the pan and onto a cutting board. Sprinkle with flaky salt, if desired, before cutting into squares.