Preheat oven to 350°F (175°C) and grease an 8x8 inch square baking pan.
In a medium bowl, whisk together 1/2 cup melted butter and 1 cup sugar until well combined.
Add 2 eggs and 1 teaspoon vanilla; whisk until the mixture is smooth and glossy.
Gently fold in the cocoa powder, flour, and salt until no dry streaks remain.
Spread the brownie batter evenly into the prepared pan and bake for 20 minutes.
While the base bakes, prepare the topping by whisking 3 eggs, corn syrup, 1/2 cup sugar, 2 tablespoons melted butter, and 1 teaspoon vanilla in a separate bowl.
Stir the chopped pecans into the topping mixture.
Remove the brownie pan from the oven and slowly pour the pecan mixture over the partially baked brownie layer.
Return the pan to the oven and bake for an additional 25 to 30 minutes until the pecan topping is set and no longer jiggles in the center.
Transfer to a wire rack and cool completely in the pan for at least 2 hours to allow the layers to set before slicing.