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In a large heatproof mixing bowl or the bowl of a stand mixer, briefly whisk together the sugar, egg whites, and salt.
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Place the bowl over a saucepan of simmering water (you should just barely see some lazy bubbles coming up), and whisk constantly until the mixture measures 161°F on a digital thermometer, about 5 to 10 minutes.
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Remove the bowl from the heat, and beat the meringue with your mixer's whisk attachment on medium-high speed until stiff, about 15 to 20 minutes. The meringue should have cooled down by this point; the bowl should feel just slightly warm to the touch or room temperature.
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Decrease the mixer speed to medium, and with it running, add the butter a few tablespoons at a time. Make sure each portion of butter is completely incorporated before adding the next, about 1 minute between each addition.
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Once about half the butter is in, stop and scrape any sticky residue from the sides and bottom of the bowl. Whisk to combine, then finish adding the butter, a few tablespoons at a time. Don’t be concerned if at any point during the process of adding the butter, the mixture seems to “break” or become greasy and lumpy. Continue adding the butter, a few tablespoons at a time, until it’s all incorporated. Once added, increase the mixer speed to high and whisk for 1 to 2 minutes or until the buttercream is light, fluffy, and smooth.
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Beat in the flavoring of your choice. (See “tips,” below for options.)
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If the frosting seems soft, chill it in the refrigerator for 15 minutes before using.
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Store the Swiss Meringue Buttercream in the refrigerator for up to one week or in the freezer for up to six months. Bring refrigerated or frozen buttercream to room temperature and re-whip in a mixer before using.
