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1
Beat egg whites with cream of tartar and salt in a large bowl with an electric mixer at medium speed until they just hold soft peaks.
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2
Add maple sugar 1 teaspoon at a time, beating, and continue to beat until whites just hold stiff peaks.
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3
Boil syrup in a small heavy saucepan over moderate heat, undisturbed, until it reaches soft-ball stage (registering 238 to 242F on thermometer), about 3 to 7 minutes.
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4
Immediately remove from heat and slowly pour hot syrup in a slow stream down side of bowl into egg whites, beating constantly at high speed.
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5
Beat meringue, scraping down side of bowl occasionally with a rubber spatula, until meringue is cool to the touch, about 6 minutes.
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6
(It's important that meringue be fully cooled before proceeding.)
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7
With mixer at medium speed, add butter 1 tablespoon at a time, beating well after each addition.
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8
(If buttercream looks soupy after some butter is added, meringue is too warm: Chill bottom of bowl in a larger bowl of ice and cold water for a few seconds before continuing to beat in remaining butter.)
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9
Continue beating until buttercream is smooth.
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10
(Mixture may look curdled before all butter is added but will come together before beating is finished.)
