Wilton Chocolate Buttercream... how much cocoa do I add?

Eh, I just keep adding cocoa until I get the color I want, then I put in the rest of the powdered sugar. Maybe half a cup or so. I've never done the Wilton class, but that always works for me. Since you're doing black, there's no reason you can't use real butter instead of shortening (blech) and butter flavor (extra blech). Honestly, butter doesn't add *that* much color to buttercream. I use butter for my whites too.