Source: Martha Stewart Weddings
- 2 1/2 cups sugar
- 10 large egg whites
- 4 cups (8 sticks) unsalted butter, cut into small pieces, room temperature
- 1 teaspoon pure vanilla extract
In the heat-proof bowl of an electric mixer, combine the sugar and egg whites. Place bowl over a pot of simmering water, and whisk constantly until the mixture feels hot to the touch and the sugar is dissolved, about 3 minutes. Transfer bowl to the mixer stand. Using the whisk attachment, beat the mixture on high speed until cooled, about 15 minutes.
Beat in the butter, one piece at a time, beating until incorporated. Beat in vanilla. Use immediately or store in an airtight container in the refrigerator.