In a large mixing bowl, beat egg yolks on until light and fluffy, about 5 minutes.
In a medium saucepan, bring the maple syrup to a boil over medium to medium-high heat. Let cook until it registers 240 degrees on a candy thermometer, about 15 minutes. Remove from heat.
With the mixer running, slowly pour syrup down the side of the bowl in a slow, steady stream, until completely incorporated, about 1 minute. Continue beating until bowl is just slightly warm to the touch, 4 to 5 minutes. Add butter, one piece at a time, and mix until thoroughly incorporated and the frosting is fluffy, about 4 minutes more.