Source: Martha Stewart's Cupcakes
- 6 large egg yolks
- 2 cups pure maple syrup, preferably grade A dark amber
- 1 pound (4 sticks) unsalted butter, chilled and cut into small pieces
In the bowl of an electric mixer fitted with the whisk attachment, beat egg yolks on high speed until light and fluffy, about 5 minutes.
Meanwhile, pour maple syrup into a medium saucepan. Place pan over medium-high heat; bring syrup to a boil. Cook syrup until a candy thermometer registers 240 degrees (just above soft-ball stage), about 15 minutes.
Remove the saucepan from the heat. While the electric mixer is running, pour the syrup in a slow, steady stream down the side of the mixing bowl into the egg-yolk mixture (it is essential that the syrup touches the side of the bowl as you pour it in so the sugar will be very evenly incorporated and not splatter onto the sides of the bowl) until the syrup has been completely incorporated, about 1 1/2 minutes. Beat until the bowl is just slightly warm to the touch, 5 to 6 minutes.
Add butter, one piece at a time, until all of it has been completely incorporated and the frosting is fluffy, about 4 minutes more. Use immediately.