Prepare the brownies. Preheat the oven according to the directions on the brownie mix box. Prepare the brownie batter using the ingredients listed on the package, then spread it evenly into the recommended baking pan.
Bake the brownies according to the package directions, then begin preparing the pecan topping while they are in the oven.
Make the pecan pie topping. Add the butter, brown sugar, and corn syrup to a medium saucepan over medium heat. Cook, stirring frequently, until the butter has melted and the brown sugar is dissolved.
Stir in the cornstarch until completely combined. Allow the mixture to gently come to a boil, then remove it from the heat. Stir in the salt, cinnamon, and pecans until the pecans are completely coated in the brown sugar mixture.
Once the brownies have finished their initial bake, remove them from the oven. Reduce the oven temperature to 325°F. Spoon the pecan pie topping evenly over the brownies, spreading it all the way across the top.
Return the brownies to the oven and bake for an additional 15 minutes, or until the pecan topping is bubbling and beginning to set.
Allow the brownies to cool completely before slicing so the pecan topping has time to firm up. Cut into 9 large brownies or smaller squares and serve.