Preheat the oven to 350°F / 175°C. Line a baking sheet with parchment paper or with a silicone mat.
In a medium sized microwave safe bowl, melt the butter in 20 second intervals, stirring after each, until it has just completely melted. Then, whisk in the cocoa powder.
Add the light brown sugar to the butter and whisk until combined.
Add in the egg white and vanilla, whisking until thoroughly combined.
Add in the flour, baking powder, baking soda, and salt. Lightly fold together using a rubber spatula until the flour is almost completely combined. Then, add in the chopped chocolate (save some for topping the cookies if desired), and fold until no flour streaks remain.
Use a 3 tbsp. cookie scoop to dish out the cookies and evenly space them out on the baking sheet. Top with extra chocolate if desired. Bake for 10 minutes, or until the edges are golden and the middles of the cookies are almost completely set.
Immediately after taking the cookies out of the oven, you can take a bowl that is slightly larger than the cookies and swirl it around over the cookies to make them a little more uniform and circular.
Serve, and enjoy!