1. Preheat the oven to 350°F (175°C). Line an 8×8-inch metal baking pan with parchment paper, leaving an overhang on two sides for easy lifting. Lightly grease the parchment and any exposed sides of the pan.

2. In a medium saucepan or heatproof bowl, melt the butter until smooth. Whisk in the cocoa powder until glossy and no lumps remain. Set aside to cool until warm but not hot.

3. In a large mixing bowl, whisk together the granulated sugar, light brown sugar, and eggs until the mixture looks slightly thickened and lighter in color, about 1–2 minutes. Whisk in the vanilla extract and espresso powder, if using.

4. Slowly pour the warm butter–cocoa mixture into the egg mixture, whisking constantly until the batter is smooth, dark, and shiny.

5. Sprinkle the flour and fine sea salt over the top of the batter. Use a spatula to gently fold just until no dry streaks of flour remain, being careful not to overmix.

6. Fold in the chocolate chips or chopped chocolate until evenly distributed. The batter will be thick and glossy.

7. Scrape the batter into the prepared pan and spread it into an even layer, smoothing the top with a spatula. Sprinkle with flaky sea salt if desired.

8. Bake for 25–30 minutes, or until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs but no wet batter.

9. Place the pan on a wire rack and let the brownies cool completely in the pan. Use the parchment sling to lift the cooled slab onto a cutting board, then slice into 9–16 squares, wiping the knife between cuts for clean edges.

10. Serve at room temperature or slightly warmed, and store any leftovers in an airtight container at room temperature for up to 4 days or freeze for longer storage.