Pour the granulated sugar on a plate or shallow dish like a pie plate and set aside. In a medium bowl, stir together the flour, cocoa, baking soda and baking powder.
In a large bowl, melt 10 tablespoons of the butter in the microwave. Take care not to overheat; microwave the butter until just melted. Stir in the remaining 4 tablespoons butter until melted. Allow the butter to cool for 5-10 minutes (it should be about 90-95 degrees on an instant read thermometer).
Whisk the brown sugar, vanilla and salt into the melted butter until the mixture is completely smooth. Whisk in the egg and egg yolk until smooth. Stir in the flour mixture until just combined.
Roll the dough into balls, about 2 tablespoons for each cookie. Roll the balls in the granulated sugar and place on lightly greased or lined (with parchment or silpat liners) baking sheets. Using the bottom of a glass, flatten the cookies to about 1/4-inch thick, more or less. Sprinkle the cookies with a bit more granulated sugar (just a pinch per cookie).
Bake the cookies, one sheet at a time, until they have puffed and have cracks running through the top, about 12-14 minutes. Don't overbake or the cookies will be dry. Transfer the cookies to a baking rack to cool completely.