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 to room temperature and beat until smooth with the machine.
Beat in flavoring, a little at a time, and continue beating until buttercream is smooth, about 2 minutes longer (always flavored buttercream immediately before using it.)
Recipe courtesy of Gourmet Magazine