Any help with icing a square cake and getting sharp corners????????
And How to ice a chocolate cake and not get crumbs??????
Both of your questions are covered in the first section.... http://cakecentral.com/cake-decorating-ftopict-654103.html
It really helps if you have a square corner to start with. Most readily available pans have rounded corners and edges and you may have to work more to get those edges sharp. Magic Line pans (and some others...I just love ML!) have super-sharp corners/edges and make it a lot easier.
Also, having a good medium (or even a little stiffer) consistency icing helps. Build up the icing on the corner, and use a hot bench scraper to pull the icing away from the corner. Hope that makes sense! It's much easier to put on lots of icing, and take away what you don't need rather than trying to build the corner up little by little to get it square.
YouTube has a couple of videos that help, and Sharon Zambito's "Perfecting The Art of Buttercream" also shows you how to do it.
Regarding the chocolate cake, I usually crumb coat chocolate and red velvet, just because they have such brightly colored crumb. I apply a very thin layer of buttercream just to seal in the crumbs, let it crust well, then ice my cake with a regular coat of buttercream.