Heat oven to 350°F. Line a 9x13-inch pan with parchment, leaving overhang, and lightly grease.
Whisk melted butter, granulated sugar, and brown sugar until shiny and thick.
Whisk in 3 eggs, one at a time, then stir in 1 teaspoon vanilla.
Fold in flour, salt, cocoa, and baking powder just until no dry spots remain.
Spread batter in pan. Bake 15–18 minutes until edges look set. Cool 5–10 minutes.
Make topping: whisk corn syrup, brown sugar, melted butter, 2 eggs, remaining vanilla, salt, and flour until smooth.
Stir in chopped pecans (and bourbon if using).
Pour topping over the warm brownie base and spread evenly to the corners.
Bake 25–30 minutes more, until the topping is set and bubbles at the edges.
Cool completely in the pan. Chill 30 minutes for cleaner slices, then lift and cut into squares.