Preheat your oven according to the temperature on your brownie box. Grease a 13x9-inch baking pan well.
In a small saucepan, whisk together the sugar, corn syrup, eggs, butter, and vanilla extract until fully combined.
Mix the brownie batter according to the package directions and pour into the prepared pan. Bake for about half the time listed on the box, until set across the whole pan.
Cook the sugar mixture over medium-high heat, stirring constantly, for about 15 minutes, until it coats the back of a spoon. Stir in the chopped pecans and cook 2-3 more minutes until pourable.
Pour the warm pecan filling over the pre-baked brownie layer and spread evenly. Bake another 20-30 minutes, until the center only slightly wobbles.
Let the pan cool completely to room temperature before cutting into squares.