Preheat oven: Set to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang, and lightly grease.
Make brownie batter: Whisk melted butter, sugar, and cocoa powder until smooth. Add eggs and vanilla, whisking until combined. Fold in flour and salt. Stir in chocolate chips, if using. Spread batter evenly in the pan.
Bake brownie layer: Bake for 15 minutes until just set. Remove from oven.
Prepare pecan topping: Whisk brown sugar, corn syrup, melted butter, egg, vanilla, and salt until smooth. Stir in pecans.
Add topping: Pour pecan mixture over the brownie layer, spreading evenly.
Finish baking: Bake for 20-25 minutes until the topping is golden and slightly bubbly. Cool completely before slicing.
Serve: Cut into 16 squares and enjoy warm or at room temperature.