There is an awesome video on you tube the lady shows. It's all about getting the buttercream on right she does a few thin layers of icing and freeze between each one. I always did one big thick layer and got rounded edges. I have also seen people put walls up of fondant on each side and top and royal icing or buttercream along the seam. Sorry don't know her name search how to smooth buttercream cake. I think that is what I searched.
The sharp edges are most commonly achieved by using ganache under fondant instead of buttercream. Hth
Meringue buttercreams. Ganache is making a splash lately, but meringue buttercreams are much more widly used in the US, and you get just as sharp corners.
J
See this video from "Inspired by Michelle"
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