Preheat oven to 350°F. Line a baking sheet with a silicone baking mat or parchment paper and set it aside. Add the ¼ cup of granulated sugar for rolling to a bowl and set it aside.
Cream together the softened butter, light brown sugar, and granulated sugar for about 1-2 minutes until light and fluffy. Add the egg, egg yolk, and vanilla extract.
Add in the flour, cocoa powder, baking soda, and salt. Mix until everything is combined and a thick dough forms.
Using a small cookie scoop, scoop dough and roll into balls. Once rolled into balls, roll into the granulated sugar, and place on the prepared baking sheet, about an inch and a half apart.
Bake the cookies for 8-9 minutes. The indents will have puffed up a bit, so you will have to slightly press down them down again with the ¼ teaspoon. Allow them to rest about 5 minutes, then transfer cookies to a cooling rack to finish cooling completely. Repeat until all of the cookies are baked.