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1
In the heatproof bowl of an electric mixer set over a saucepan of simmering water, combine the egg whites and sugar.
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2
Cook, whisking constantly, until the sugar has dissolved and the mixture is warm to the touch (about 160F).
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3
Attach the bowl to the mixer fitted with the whisk attachment.
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4
Beat the egg-white mixture on high speed until it holds stiff (but not dry) peaks.
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5
Continue beating until the mixture is fluffy and cooled, about 6 minutes.
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6
Switch to the paddle attachment.
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7
With the mixer on medium-low speed, add the butter several tablespoons at a time, beating well after each addition.
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8
(If the frosting appears to separate after all the butter has been added, beat on medium-high speed until smooth again, 3 to 5 minutes more.)
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9
Beat in vanilla.
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10
Beat on lowest speed to eliminate any air bubbles, about 2 minutes.
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11
Stir with a rubber spatula until frosting is smooth.
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12
Follow instructions for Swiss Meringue Buttercream, omitting the vanilla extract and stirring in 3/4 cup Lemon Curd (page 390) with a rubber spatula at the end.
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13
Follow instructions for Swiss Meringue Buttercream, omitting the vanilla extract, decreasing the sugar to 1 cup, and adding 8 ounces melted best-quality bittersweet chocolate after the butter has been incorporated (let chocolate cool slightly before mixing in).
