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1
Bring a few inches of water to a boil in a saucepan that can hold a heatproof stand mixer's bowl above the water.
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2
Add the sugar, lemon juice, salt and egg whites to the mixing bowl, and whisk together by hand.
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3
Set the bowl above the boiling water and whisk until the mixture is warm to the touch and the sugar completely dissolves.
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4
Transfer to the stand mixer fitted with the whisk attachment and beat at medium-high speed until cool and the whites hold stiff peaks, 10 to 15 minutes.
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5
Toss in a couple of chunks of butter at a time, making sure the pieces are incorporated before adding more.
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6
After all the butter is added, continue whisking on medium-high speed.
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7
The mixture will deflate and appear curdled.
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8
Continue whisking until the buttercream comes back together to form a smooth and spreadable consistency.
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9
Slowly drizzle in the optional flavoring while beating until smooth and light.
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10
(If the frosting is very soft or begins to break, refrigerate until set but still spreadable and then beat until light before using.)
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11
Chocolate:
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12
Remove the buttercream from the stand mixer into a large bowl and, in three batches, fold in 6 ounces melted and cooled bittersweet chocolate.
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13
Citrus:
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14
Stir in the zest of half a lemon or orange.
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15
Boozy:
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16
Stir in 2 tablespoons coffee-flavored liqueur and 1 tablespoon vodka.
