Could I add cocoa powder to this?

If you meant you like the taste of real cocoa, just use chocolate that has a cocoa content of 55% or higher. You can also use dark chocolate which usually has about the same high content. Most chocolate such as Hersheys and Nestles have less than 10% cocoa content. Real cocoa is expensive, so if the powder/chocolate is relatively cheap you can be sure it is lower in cocoa content.