Set the oven to the temperature specified on your brownie package and let it preheat. The oven should be fully heated before the batter goes in so the chocolate layer begins setting evenly.
Use a small saucepan to thoroughly combine the sugar, corn syrup, eggs, butter, and vanilla, then cook over medium-high heat while stirring constantly. The mixture will begin loose and become smoother as the butter melts into it.
Prepare the brownie batter using the ingredients and mixing directions printed on the box. Keep mixing only as directed so the batter retains the intended consistency.
Grease a 13x9-inch pan, spread the batter evenly inside, and bake for 20 minutes. This first bake gives the chocolate layer a head start without finishing it before the topping goes on.
Keep stirring the saucepan mixture until it leaves a coating on a spoon, about 15 minutes, then incorporate the pecans and cook for another 2 to 3 minutes. The finished filling should resemble runny oatmeal, with enough body to suspend the nuts while remaining pourable.
Take the partially baked brownies out of the oven and distribute the cooked pecan filling across the entire surface. Spread it gently into an even layer that reaches the corners.
Return the pan to the oven for 25 to 30 minutes, checking for a small wobble at the center of the topping when you gently move the pan. The edges should look set, and the middle should tremble slightly rather than ripple like liquid.
Allow the brownies to reach room temperature completely before cutting them into squares. The layers become more stable as they cool, helping the topping stay attached when sliced.