Directions:

1. In a small bowl whisk together the granulated sugar and egg whites to make a thick slurry. Place the bowl over a small saucepan of simmering water and whisk occasionally until the mixture is hot to the touch. It will thin out a bit as the sugar melts.
2. Scrape the sugar-egg white mixture into the bowl of a stand mixer fitted with a whisk attachment (or into a medium bowl and use an electric hand mixer). Whip on medium-high speed for 6 to 8 minutes, until the mixture cools. Add the butter to the sugar-egg white mixture and whip on medium speed until the butter is thoroughly incorporated and the whole mixture is smooth and satiny.
3. The frosting can be made ahead and stored in an airtight container for up to 3 days at room temperature or up to 2 weeks in the refrigerator. Before using, place the frosting in the bowl of a stand mixer fitted with a paddle attachment and re-mix until the frosting smoothes out again.