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 temperatur e 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.)

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 raspberry puree made by cooking down and straining a 10-ounce package of frozen raspberries