One of the most noticeable differences between a shop-bought cake and one made at home is the icing. Commercial icing has a relatively bland flavour, because it is usually made with shortening or margarine rather than butter. Real homemade buttercream has a distinctive and unmistakable richness that makes any cake better. Most recipes call for unsalted butter, but salted butter can be used when needed.
Icing plays an important role in cake decorating, for both home and professional bakers. There are many ways to cover a cake, but conventional buttercream is the most versatile. The fat in the icing provides a seal over the cake, keeping out the air and protecting it from staling in the same way a tight plastic wrapping would. The icing also adds flavour and richness to the cake, and also provides lubrication for the diner's mouth. Buttercream also makes an attractive surface that can be coloured and piped to provide decorative accents.
Buttercream icing combines sugar and butter in a number of ways, and adds vanilla or other flavourings as needed to match the cake. The simplest version creams butter together with icing sugar, making a topping that is light from the air creamed into the butter. Lighter versions whip egg whites into a foamy, sweet meringue and then whip in softened butter once the meringue is stable. In either case, the icing must be applied when it is soft enough to spread easily and stick to the cake, yet firm enough to hold its shape.
Almost every recipe calls for icing to be made with unsalted butter. Unsalted butter has a clean, neutral creamy flavour, which is an excellent complement to most varieties of cake. Its neutrality ensures that any added flavours will be as pure as possible, putting the baker in control of the end product. Pure butter has the added advantage of melting at body temperature, which means it doesn't leave a greasy film in the mouth, as shortening does.
Most people keep salted butter in the house for table use, and at times only salted butter might be available to make icing. The end result will still be very palatable, though there are a few considerations to bear in mind. If the recipe calls for salt at any stage except whipping the egg whites, leave it out. If possible, use the resulting buttercream on cakes with a distinct flavour of their own, like chocolate cake or fruitcake. This will help mask the salt. So will a small amount of lemon juice added to the icing.