What's YOUR secret to black icing?!?!

My mil told me her friend has told her what ingredients to buy to make black icing, but we're having issues with dh's parents atm, so I was wondering how YOU would make it!? I'm having a go at making Jensen's bday cake early so when it comes time, I'll have it right on the day!!

thanks in advance!
(oh btw, I'm on a diet, but I'll take any suggestions!!)