Preheat oven to 325 degrees F. Line 2 baking sheets with parchment paper or silicone liners; set aside briefly.

In a medium microwaveable bowl, combine the butter and chopped chocolate bar. Microwave for 30 seconds, stirring until melted and smooth (you may need to microwave an additional 15-20 seconds). Cool for about 5 minutes.

Meanwhile, in the bowl of a stand mixer, combine the brown sugar, eggs, vanilla extract and espresso powder with the paddle attachment until combined. Pour in the melted chocolate and beat to combine. Lastly, add in the flour, cocoa powder, baking powder, baking soda, and salt and mix on low speed until combined. Batter will be thick and sticky. Let set at room temperature undisturbed for 10 minutes until thickened.

Place the confectioners' sugar in a small bowl. Drop rounded Tablespoonfuls of dough into the confectioners' sugar and roll the ball of dough around to fully coat. Place 1-2" apart on the baking sheets. Bake for approximately 10-12 minutes, rotating pans halfway through baking time to ensure even baking. Cookies may appear slightly underdone in the center; this is okay as they will continue to set up as they cool. Cool completely on the baking sheets to set before serving.