This version of this icing goes on our Dreamy Coconut Wedding Cake.
In the heatproof 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.
Reduce mixer speed to medium, and begin to add the butter, a few bits at a time, incorporating well after each addition. After all the butter is added, add the vanilla extract; combine. Store buttercream in an airtight container until ready to use.
Note: Raw eggs should not be used in food prepared for pregnant women, babies, young children, the elderly, or anyone whose health is compromised.