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1
In the bowl of a standing electric mixer fitted with the whisk attachment, mix egg yolks on high speed until pale and thick, about 5 minutes.
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2
Meanwhile, bring maple syrup to a boil in a saucepan over medium heat; clip a candy thermometer to side of pan.
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3
Cook until syrup registers 240F, about 15 minutes.
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4
Remove from heat.
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5
With mixer on medium speed, carefully pour the syrup in a slow, steady stream down the side of the bowl until it is completely combined, about 1 1/2 minutes.
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6
Continue mixing until the bottom of the bowl is only slightly warm to the touch, 5 to 6 minutes.
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7
Add the butter, a few tablespoons at a time, mixing until completely incorporated after each addition.
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8
Once all butter has been added, scrape down sides of bowl with a flexible spatula, and continue beating until buttercream is fluffy, about 4 minutes more.
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9
Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month.
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10
Before using, bring to room temperature and whisk on low speed until fluffy again, about 5 minutes.
