Does it have to be yellow? If not, I'd make chocolate ganache. It doesn't get much easier than this:
16 oz bittersweet chocolate pieces
16 oz heavy cream
Heat heavy cream til frothy, pour over chocolate, let stand 2 minutes then combine thoroughly.
Bring to a touch cooler than room temp then whip it into a frosting. People will be amazed at how delicious it is.
Danielle, Nick, Gigi ('08) and William ('11)