Preparation

• Arrange a rack in the middle of the oven and heat the oven to 325°F. Generously coat a 9-inch square baking pan with cooking spray. Line the bottom and sides of the pan with 2 pieces of overlapping parchment paper, arranging them perpendicular to each other so that the paper extends over all 4 edges by a few inches.
• Prepare the batter from 1 (18-ounce) box 'Family'-sized boxed brownie mix according to package directions. Transfer to the baking dish and smooth out 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. Meanwhile, prepare the pecan pie topping.
• Coarsely chop 2 cups pecans. Place 4 tablespoons unsalted butter, ⅓ cup packed light brown sugar, ⅓ cup light corn syrup, 2 tablespoons heavy cream, ¾ teaspoon vanilla extract, and ½ teaspoon kosher salt in a medium saucepan. Bring to a boil over medium heat, stirring occasionally. Simmer, stirring constantly, for 2 to 3 minutes. Take the pan off the heat. Add the pecans and stir to combine until evenly coated.
• When the brownies are ready, remove the pan from the oven. Increase the oven temperature to 350ºF. Evenly spoon the filling over the brownies. Return the pan to the oven. Bake until the pecan pie topping is bubbling all over, particularly in the center of the pan, 25 to 30 minutes.
• Place the pan on a wire rack and let the brownies cool to room temperature, about 1 ½ hours. Grasping the excess parchment, lift the brownie slab out of the pan and place on a cutting board. Sprinkle with flaky salt if desired before cutting.