Preheat the oven to 350°F (177°C). Line a full sheet pan (or two half sheet pans) with parchment paper. In a small bowl, whisk together the all purpose flour, cocoa powder, espresso powder (if using), baking powder, and Kosher salt. Set aside.

Fill a small pot with 1-2 inches of water and bring it to a gentle simmer. Add the dark chocolate and unsalted butter cubes to a heatproof mixing bowl. Set the bowl over the simmering water, making sure that the water doesn't touch the bottom of the bowl. Stir constantly, just until the chocolate mixture is smooth and completely melted. Let it cool slightly.

Meanwhile, add the dark brown sugar, granulated sugar, and egg to a large mixing bowl. Using a whisk attachment, whip the egg and sugars together for a full 6-7 minutes, until it's lighter in color, nearly doubled in volume, and looks similar to cake batter.

With the mixer running, slowly drizzle the melted chocolate into the sugar-egg mixture until smooth. Scrape down the sides of the bowl every once in a while to make sure everything is well-combined. Stir in the vanilla extract. Then, fold the dry ingredients into the batter, just until the dough is combined.

Using a #100 cookie scoop or teaspoon, quickly scoop the dough into small balls, and place them about one inch apart on the prepared sheet pan.

Bake the cookies for 5-6 minutes, flipping the pan halfway through, just until the edges are set and the centers are still a bit underdone. The tops will still be a little shiny. Let the cookies cool on the pan for about 5 minutes, and then transfer them to a wire rack to cool completely. Sprinkle the cookies with flaky salt, to taste. Enjoy!