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1
The sugar syrup and egg whites will need to be ready at roughly the same time, so preparing them will require some coordination. In a small saucepan over medium-low heat, bring the sugar and water to a simmer, swirling occasionally, and cook until the sugar dissolves. Continue simmering, without stirring, until the syrup reaches 248u00b0F on a digital thermometer, about 5 minutes.
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2
Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment, whip the egg whites and cream of tartar on medium speed until the whites hold soft peaks, 8 to 10 minutes.
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3
Immediately transfer the sugar syrup to a heatproof liquid measuring cup. With the mixer running on medium-high speed, add the hot sugar syrup to the whites in a thin, steady stream; aim to pour it into the small space between the mixer bowl and the whisk. Continue to whip the mixture until thick and shiny and the outside of the bowl feels cool to the touch, about 10 minutes.
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4
Decrease the speed to medium and, with the mixer running, add the butter one piece at a time. The mixture won't resemble perfect buttercream until the last of the butter has been added. If it's looking deflated, wet, or broken during mixing, make sure the butter is at room temperature and slow down the additions, adding the next piece only after the last one has been fully incorporated. Add the vanilla and beat on medium speed until well combined, about 1 minute, until the texture is similar to mayonnaise.
