Preheat oven to 350°F (175°C). Line a 9x13-inch pan with parchment, leaving overhang.
Whisk melted butter and sugar until smooth. Add eggs one at a time, then stir in vanilla.
Sift in flour, cocoa, and salt. Fold gently into a thick batter.
Spread batter in pan and bake for 20 minutes—set edges, soft center.
Mix corn syrup, brown sugar, eggs, melted butter, and vanilla. Stir in pecans.
Pour topping over hot brownie base. Gently spread without mixing layers.
Bake 30–35 minutes more, until golden and just set. Add 5 minutes if jiggly.
Cool completely. Chill 1 hour for cleaner slices, then cut into squares.