The recipie Buttercream - Classic Recipe on this page is phenomenal.
I just made it
(after two failed attempts - follow DIRECTIONS Ammy, sheesh - note to those who don't know better, fat and oil causes egg whites to fail miserably at the whole fluffing/stiffening thing when you whip them, so if there are even traces of butter in the bowl, or if you add something with fat in it (melted chocoalte) before the eggs are as whipped and stiff as you want them - you will RUIN EVERYTHING)with about 3 oz melted bittersweet chocolate (added AFTER the butter), and probably almost a cup of cocoa powder. (added at the very end) And it's.. god it's good. I ate so much I feel ill. I regret eating as much as I did, that's for sure.. but it's super awesome frosting, I must tell you. And so very smooth and beautiful too!