Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper or just have ready two ungreased baking sheets.
If you haven't already, melt your butter and allow it to cool. The best way to do it is to melt 10 tablespoons of the butter completely, then add the remaining 4 tablespoons of butter and stir until melted.
Whisk flour, cocoa, baking powder, baking soda and salt together in a bowl.
Whisk together brown sugar, vanilla, and melted butter until no lumps remain, scraping down bowl as needed. Whisk in egg and yolk until smooth.
Stir in flour mixture until just combined.
Spread granulated sugar in a shallow bowl; set aside.
Working with 2 tablespoons dough at a time, roll into balls. Then roll in granulated sugar to coat; space dough balls evenly on prepared sheets. Alternatively, you can scoop dough with a large cookie scoop.
Using bottom of dry measuring cup, press each ball until 2 inches in diameter. Evenly sprinkle remaining granulated sugar over cookies.
Bake 1 sheet at a time until cookies are slightly puffy and edges have begun to set (cookies will look raw between cracks and seem undone), about 15 minutes, rotating sheet halfway through baking.
Let cookies cool on sheet for 5 minutes, then transfer to wire rack. Let cookies cool completely.
I used Ghirardelli natural but I think a Dutch chocolate such as Guittard Cocoa Rough would also be good.