I also found the black cocoa at a Amish store and it was really cheap!
I should probably mention-you can sub all or half of the shortening with real butter, and you can also use half and half or heavy cream in place of the milk (that will make it creamier and smooth out nicer).
Oh, add black color if it's not as dark as you need it. It will darken more the next day. I like AmeriColor-it seems to deepen the best with the least bitterness.
also, instead of milk you can use strong brewed coffee to make it even darker
Does this recipe harden for sugar cookie decorating?