For many people, the frosting is the best part of a cake or cookie. I classify frosting into six types: buttercream, cooked, whipped cream, royal icing, ganaches, and glazes.
Butter or margarine is combined with powdered sugar, flavorings, and liquid. Powdered sugar is used because it dissolves easily during beaten and contains a small amount of cornstarch which helps stabilize the frosting. There are buttercream frostings that use granulated sugar, but these take a long time to make and can be difficult. Classic buttercream usually involves making a custard, then beating butter into it until a frosting consistency is reached. I prefer simple buttercream frostings, beating softened butter with powdered sugar, liquids and flavorings. Beat the frosting longer than you think is necessary for the smoothest and fluffiest results. It's really not possible to overbeat this type of frosting.
Seven-minute frosting is the classic cooked frosting. Egg whites and sugar, along with flavorings, are combined in the top of a double boiler. The mixture is gently heated while continuously beating with a mixer. Please use a mixer if you choose this type of frosting; you can beat it by hand with an eggbeater, but it's really difficult. As the mixture cooks, a meringue forms, which is stabilized as it grows because it is being heated. Cooked frostings hold their shape because the egg white proteins have been coagulated by the heat. These frostings must be cooked to a temperature of 140 degrees for safety. These frostings are delicate and can be absorbed into the cake if not eaten the first day. If you prefer, you can use meringue powder to make seven-minute frosting without fear of food poisoning from eggs.
Powdered sugar, flavorings, and whipped cream make whipped cream frostings - what could be simpler? Again, the cornstarch in the powdered sugar helps stabilize the frosting. It is possible to overbeat this type of frosting, so just beat until firm peaks appear when you stop beating and lift the beater straight up from the frosting. Cakes, cupcakes, shortcakes, and cookies with this type of frosting must be refrigerated.
This icing is used for decorating cakes and cookies. You can make it from scratch, using powdered sugar, egg whites, and liquid, but I prefer using meringue powder, which you can buy at bakery supply stores and even some grocery stores. The meringue powder is combined with a liquid, then usually tinted with food coloring. The proper consistency for royal icing is about the consistency of pancake batter. It should flow easily since it's usually used in pastry bags with decorating tips, but set quickly so the design holds.
This fancy term is simply chocolate melted with heavy cream. This frosting makes a beautiful shiny glaze on cakes and cookies. If you chill a ganache, beat it until it's fluffy and stiff, then form it into balls, you'll end up with truffles. You can also chill and beat a ganache and use the fluffy result to quickly frost a layer cake.
Glazes are the simplest frostings. Powdered sugar is combined with a liquid to form a thin consistency. Glazes are usually poured or drizzled over the tops of cakes and cookies. This forms a shiny hard crust when the glaze sets. Melted chocolate can be used as a glaze on its own.