Preheat your oven to 350 degrees, and line a baking sheet with parchment paper. Set aside.
In a large bowl, beat together the peanut butter, sugar, and brown sugar until combined. Add in the egg yolk, vanilla extract, cocoa powder, baking soda, and salt. Beat until combined.
Scoop your cookie dough onto the baking sheet with each cookie dough ball about 2-tablespoons in size. Press the dough down just slightly, and sprinkle on the coarse sugar.
Bake for 7-9 minutes, or until puffy and set. Cool completely on a wire rack. Enjoy!