One of the best-tasting buttercreams is also the easiest to prepare. See the end of the recipe for flavoring possibilities.
Place egg whites, sugar and salt in medium-sized heatproof mixing bowl over a pan of simmering water and whisk gently and constantly until egg whites are hot (about 140 degrees) and sugar is dissolved, about 3 to 4 minutes. Remove from heat and whip by machine until thick and cooled, about 5 minutes. Beat in butter and continue beating until buttercream is smooth and spreadable. Use immediately or refrigerate, covered, for up to 5 days. Before using, bring buttercream to room temperature and beat smooth by machine. Beat in flavoring, a little at a time, and continue beating until buttercream is smooth, about 2 minutes longer (always flavor buttercream immediately before using it).
Flavorings: Liqueur: 2 to 3 tablespoons liqueur, such as Grand Marnier or dark or light rum. Lemon: 2 to 3 tablespoons lemon juice. Coffee: 3 tablespoons instant coffee (espresso, if possible) dissolved in 2 tablespoons water, coffee or rum. Raspberry: 3/4 cup thick raspberry puree made by cooking down and straining a 10-ounce package of frozen raspberries. Chocolate: 6 ounces chocolate melted with 4 tablespoons water, coffee or milk and cooled.