Preheat oven to 325°. Grease a 9"x9" baking pan with cooking spray and line with parchment. Prepare brownie batter according to box instructions. Bake brownies until slightly underbaked and a tester inserted into the center comes out with a few moist crumbs attached, about 20 minutes.
Meanwhile, in a medium bowl, whisk corn syrup, brown sugar, butter, molasses, vanilla, and salt until smooth. Whisk in eggs until combined. Stir in pecans until coated.
Increase oven temperature to 350°. Top brownies with pecan mixture, evenly spreading. Continue to bake until pecan topping is springy to touch and thickened, 40 to 45 minutes.
Let brownies cool to room temperature. Refrigerate until cold and set, about 1 hour. Drizzle with caramel sauce (if using).
