This recipe is used to achieve a smooth buttercream finish on cakes, especially using the Viva paper towel method. This recipe is not recommended as the layer of icing underneath fondant if you’re trying to achieve sharp edges, because the shortening used in this recipe won’t allow the buttercream to harden in the freezer well enough before adding your layer of fondant.  See the Non-crusting American Buttercream recipe for that application.