Bakers, I need your help with a cookie recipe tweak - where to add the cocoa?

I have a great chocolate chip cookie recipe… Alton Brown’s, to be specific… and I want to tweak it into a chocolate chocolate chip cookie. But I don’t know how to add cocoa to it. How much do I use, when/where do I add it, and does it take the place of some other dry ingredient?

http://www.foodnetwork.com/recipes/alton-brown/the-chewy-recipe/index.html