Source: Martha Stewart's Baking Handbook
- 3 large egg yolks
- 1 cup pure maple syrup, best quality
- 2 sticks (1 cup) unsalted butter, cold, cut into small pieces
In the bowl of an electric mixer fitted with the whisk attachment, beat the egg yolks on high speed until light and fluffy, about 5 minutes; set aside. In a small saucepan set over medium-high heat, bring the maple syrup to a boil, and 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, until thoroughly incorporated and the frosting is fluffy, about 4 minutes more.