I have a friend who used cocoa powder in a Mr. Beer kit and I found it a little chalky in the mouthfeel, but it was likely he used too much and I know he didn't make a syrup with it first. That said, I've never used powder in a brew myself. If you do decide to use powder, try to get Dutch processed cocoa powder as I've found (when I'm baking, not brewing) that I prefer it's richer, smoother flavor to standard cocoa powder and it would be easy enough to incorporate it into your priming sugar as a chocolate syrup when you bottle. You can get it from spice stores if you have one nearby or from Amazon.
If you're doing extract or AG, I've always had a lot of luck with using Briess Chocolate and Dark Chocolate Malts as specialty grains as well as using ground up nibs in my mash if I want chocolate to be the star of the brew.
There's a great article about using cocoa in all its forms from All About Beer here
that should give you an insight into what form/method would be best suited for what you're creating.