Preheat the oven to 350°F (177°C). Line one full sheet pan with parchment paper. Set aside.

Fill a pot with about 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, ensuring 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 6-7 minutes, until it's lighter in color and nearly doubled in volume. It should look like 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 to make sure everything is well-combined. Then, stir in the vanilla extract.

In a small bowl, whisk together the all purpose flour, cocoa powder, espresso powder, baking powder, and Kosher salt. Fold the dry ingredients and wet ingredients together, until the dough is about 80% combined.

Then, add the chopped peppermint bark. Stir just until the dough is well-combined and there are no more streaks of flour.

Using a #100 cookie scoop or teaspoon, scoop the batter into small cookie dough balls. 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. Right out of the oven, place a few pieces of peppermint bark on top of each cookie, if desired. Let the cookies cool on the pan for about 5 minutes, and then transfer them to a wire rack to cool completely. Enjoy!