The simplest version of buttercream is nothing more than powdered sugar creamed with butter, margarine or shortening, with a bit of milk and vanilla added. Professional bakers use a variety of richer, more sophisticated versions with a smoother, more elegant texture. One of the lightest and most appealing is Italian buttercream. It differs from most other buttercreams in how it's made.
A buttercream frosting is made by creaming a quantity of butter or other fat with sugar and flavorings. The texture and quality of the frosting depend on the quality of its ingredients, and how they're combined. For example, mass-produced commercial icing often uses vegetable shortening in place of the butter. This results in a smooth but bland frosting that leaves a slightly greasy sensation in your mouth. That's because butter melts at body temperature, but shortening doesn't. A homemade frosting with powdered sugar is slightly gritty, but if it's made with butter it will have a rich flavor without that greasiness.
Many bakers are familiar with an old-fashioned favorite called boiled icing or seven-minute icing. There are two ways of making it. The first calls for whipping egg whites and sugar in a double boiler, over a pan of simmering water. The second variation makes a hot sugar syrup, while whipping the egg whites separately. When the egg whites reach the soft-peak stage, the hot syrup is drizzled into the eggs as they whip. It cooks the egg whites, resulting in a glossy, stable meringue. Professional pastry chefs call the first version a Swiss meringue, and the second version an Italian meringue. Italian buttercream is made from Italian meringue.
To convert the meringue to Italian buttercream, start by measuring your butter and cutting it into small pieces. Let it sit out for an hour or two before you start, so it's completely softened. Make the Italian meringue, whipping the egg whites until they come to soft peaks. Continue beating slowly as it cools to room temperature. When it's just warm to the touch, add the butter, one small piece at a time. When it's all incorporated into the egg foam, the eggs will deflate slightly, which is normal, but the finished buttercream will still be exceptionally light and fluffy. That makes it a good choice for finishing angel food or chiffon cakes.
The idea of using egg whites in frosting raises questions for some bakers about food safety. If you heat your sugar syrup to the correct temperature -- 240 degrees Fahrenheit, tested with a thermometer -- it will raise the temperature of your egg whites to a food-safe 160 F. For an additional degree of safety, you can start with pasteurized egg whites, which you can find in the refrigerated section at the supermarket. Once the icing is finished, the high sugar content acts as a preservative to inhibit spoilage, just as it does in jams and jellies.