Prepare the Baking Dish: Preheat the oven to 350°F and grease an 8×8-inch square baking dish. Line it with parchment paper, leaving some paper hanging over the sides to make the baked brownies easier to remove.
Add the Bittersweet Chocolate: Place the chopped 60% bittersweet chocolate in a large mixing bowl and set it aside for the browned butter.
Brown the Butter: Add the unsalted butter to a saucepan over medium heat. Melt and cook it as it changes from loud, bubbly activity to a quieter, foamy stage, which takes about 8 minutes. Once foamy, continue stirring until the butter becomes a rich amber color and develops a nutty aroma. Melt the Chocolate: Immediately pour the hot browned butter over the chopped bittersweet chocolate, making sure to scrape the browned bits from the bottom of the saucepan into the bowl. Stir until the butter and chocolate are combined and the chocolate has melted. Add Espresso and Sugars: Whisk the instant espresso powder into the melted chocolate mixture. Follow with the granulated sugar and packed dark brown sugar, whisking until incorporated.
Combine the Dry Ingredients: While the chocolate mixture melts and cools, whisk the whole wheat flour, Dutch-process cocoa powder, and salt together in a separate bowl. Set the dry mixture aside.
Whisk in the Eggs: Once the chocolate mixture has cooled, add the room-temperature eggs and vanilla extract. Vigorously whisk for about 1 minute, until the batter thickens slightly.
Fold in the Dry Mixture: Sift the flour and cocoa mixture into the chocolate batter to remove any cocoa lumps. Use a rubber spatula to gently fold everything together.
Add Dark Chocolate: Just before the dry ingredients are completely incorporated, fold in about half of the chopped 70–80% dark chocolate. Continue folding only until the chocolate is evenly distributed, taking care not to overmix the batter.
Fill the Pan: Transfer the brownie batter to the prepared baking dish and spread it into an even layer. Scatter the remaining chopped dark chocolate over the surface.
Bake the Brownies: Bake at 350°F for 20–25 minutes, or until the surface is crackled and glossy and the chocolate has melted into puddles.
Deflate the Brownies: Remove the pan from the oven and carefully slam it against a hard surface a few times to help deflate the brownies. This encourages the rich, fudgy texture in the center. Place the pan on a wire cooling rack.
Add Flaky Sea Salt: Sprinkle a few pinches of flaky sea salt over the warm brownies.
Cool Completely: Allow the brownies to cool completely. They can be left at room temperature for a few hours or chilled in the refrigerator to speed up the cooling process.
Check for Slicing: Lift the brownies from the pan using the parchment overhang and observe how they bend. If the center bows easily, they need additional cooling or chilling. If they bend only slightly while feeling sturdy, they are ready to slice.
Slice Cleanly: Use a large sharp knife to make swift, single-motion cuts rather than dragging the blade through the brownies. Wipe the knife clean between each cut for neat slices.
