Heat oven to 350°F. Line an 8-inch square pan with a parchment sling and lightly grease.
Whisk melted butter, granulated sugar, and brown sugar until glossy and combined.
Whisk in eggs and vanilla until the batter looks thick and smooth, about 30 seconds.
Sift in flour, cocoa, salt, baking powder, and espresso powder. Fold just until no dry streaks remain.
Fold in chocolate chips and walnuts, keeping the batter thick and cohesive.
Spread batter evenly in the pan. Drizzle chocolate syrup over the top and swirl with a knife.
Bake 26–30 minutes, rotating once, until shiny on top and a few moist crumbs cling to a tester.
Cool in pan 30 minutes. Lift out with the sling, cool 15 minutes more, then cut into squares.