Preheat the oven to 350 degrees F. Spray an 8x8-inch pan with nonstick spray, and then line the pan with parchment paper, leaving an overhang on all sides. Spray the parchment paper too.
In a medium, heatproof bowl, combine the chopped chocolate, espresso powder and 2 tablespoons of the cocoa powder; set aside.
Add the butter to a small saucepan over medium-heat and cook until the butter just comes to a vigorous simmer, about 5 minutes, stirring often. Immediately pour the hot butter over the chocolate mixture and let it sit for 2 minutes. Whisk until the chocolate is completely smooth and melted, then set it aside.
In the bowl of a stand mixer fitted with the whisk attachment (or using a hand mixer), whisk together the granulated sugar, dark brown sugar, vanilla, salt and eggs for precisely 10 minutes.
With the mixer on, pour in the slightly cooled chocolate mixture and mix until smooth, about 2 minutes.
Sift in the flour and the remaining 4 tablespoons of cocoa powder and use a rubber spatula to gently fold until just combined.
Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 20 minutes, and then remove the pan from the oven and slam it on a flat surface 2 to 3 times (this deflates the brownie slightly, giving them a more even texture and encourages that beautiful crackly top). Return the pan to the oven and bake until a toothpick inserted into the center of the brownies comes out fudgy, but the edges look cooked through, about 10 additional minutes. The center of the brownies will seem under-baked, but the brownies will continue to cook and set as they cool.
Allow the brownies to cool completely in the pan. Use the parchment paper to lift the cooled brownies out of the pan. Sprinkle the top of the brownies with the flaky sea salt, if desired, and cut into 16 bars.
Cover and store leftover brownie in an airtight container at room temperature for 3 to 4 days.