Just a bit of advice here if you plan to use cocoa powder.
For a more intense chocolate flavor, get cocoa powder from a specialty spice supplier like Penzey's or Savory Spice. The difference between what they sell and the grocery store varieties is ENORMOUS. I often buy cocoa powder to make chocolate syrup for my kids' chocolate milk, etc. It's unbelievable how good that stuff is. It's so rich that it makes chocolate milk taste more like chocolate pudding.
I've not used it in beer, so YMMV.