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1
Heat sugar and eggs Bring a saucepan filled with about 2 inches of water to a simmer.
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2
Meanwhile, combine sugar, egg whites, and salt in a large heatproof mixing bowl.
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3
Set bowl over (not in) simmering water, and whisk until whites are warm to the touch and sugar is dissolved, 2 to 3 minutes.
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4
(Heating the eggs relaxes their proteins, enabling the eggs to whip up higher and more quickly; the sugar also melts, for a silky smooth consistency.)
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5
Test by rubbing between your fingers; the mixture should feel smooth.
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6
Whip meringue Attach bowl to a standing electric mixer fitted with the whisk attachment, and beat on low speed until foamy.
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7
Add cream of tartar, and beat on medium-high speed until stiff, glossy peaks form and mixture is cooled completely, about 10 minutes.
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8
Mix in butter Reduce speed to medium-low; add butter 2 tablespoons at a time, beating to incorporate fully after each addition.
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9
Dont worry if the buttercream appears curdled at this point; it will become perfectly smooth again with continued beating.
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10
After all the butter has been incorporated, beat in vanilla.
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11
Finish Switch to the paddle attachment.
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12
Beat on the lowest speed to reduce air bubbles, 3 to 5 minutes.
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13
Let stand, covered with plastic wrap, at room temperature.
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14
(If not using the same day, transfer to airtight containers and store in the refrigerator up to 3 days or the freezer up to 1 month.
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15
Before using, bring to room temperature; beat with the paddle attachment on the lowest speed until smooth and pliable, about 10 minutes.)
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16
Follow recipe above, adding 5 1/2 ounces melted and completely cooled semisweet chocolate along with the vanilla.
