I am in need of a chocolate frosting recipe that can be used on a square wedding cake along with 3 sheet cakes. Thank you!!

Here's one that I love:

Use that recipe but add 6 oz. of melted unsweetened chocolate that's been cooled to room temp. I add it right before the sugar, it's very very good!

I use the Chocolate Decorator's Buttercream recipe from this site. I do increase the amount of chocolate to make it a little darker.
Good luck!