Preheat the oven to 350℉. Line two baking sheets with parchment paper or silpat mats.
Add the sugar to a shallow dish; set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda and baking powder.
In a large bowl, microwave 10 tablespoons of the butter until melted- about 1 minute. Remove from the microwave and stir in the remaining 4 tablespoons of butter until melted. Let the butter stand for 5 minutes.
Whisk the brown sugar, vanilla and salt into the butter until no lumps remain. Whisk in the egg and the egg yolk until smooth. Stir in the flour mixture until just combined.
Roll the cookie dough into balls (2 tablespoons per ball). Roll the balls in the sugar and space them evenly on the prepared baking sheets. Use the bottom of a drinking glass to flatten the cookies to 2-inches in diameter. Sprinkle each of the cookies with 1½ teaspoons of the remaining sugar.
Bake cookies one sheet at a time, until slightly puffy and the edges have begun to set, about 15 minutes. Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.