Preheat the oven to 350°F (180°C). Grease and flour an 8×8-inch baking pan.
Prepare the brownie batter according to the package directions. Pour half of the batter into the prepared pan and spread evenly.
1 box (18.25 ounces) brownie mix, 3 eggs, ½ cup butter, 3 tablespoons water
In a medium bowl, combine the chopped pecans, brown sugar, flour, and melted butter. Mix until well combined.
1 cup chopped pecans, ½ cup packed light brown sugar, ¼ cup all-purpose flour, ¼ cup unsalted butter
Spread the pecan mixture evenly over the brownie layer.
Spoon the remaining brownie batter on top and gently swirl the layers together with a knife—do not overmix.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out with moist crumbs.
Let the brownies cool completely in the pan before slicing into squares.