I’d love to make a chocolate eversion if this frosting. Would it be best to use cocoa powder or melted chocolate and in what amount?

German buttercream starts life as a pastry cream, before being transformed by all the butter. So you can opt to flavor either one, to your heart's desire. If you flavor the pastry cream, the butter will mute the flavor some- so that's one thing to consider. Or you can flavor the finished buttercream, exactly the same as you would any other buttercream. When I make a chocolate version, I add 8 ounces of good quality dark chocolate to the milk I make the pastry cream with, and then beat in a tablespoon of espresso powder mixed with enough hot water to make a paste at the end of the process. You could however, add both at the end if you want. Honestly, flavoring is at your command pretty much, so you can make a lot of different versions that way.