Directions

1. Combine sugar, egg whites, and salt in the heatproof bowl of an electric mixer. Set over a pan of simmering water. Whisk constantly until sugar has dissolved and mixture registers 160 degrees on an instant-read thermometer.
2. Return bowl to mixer; fit mixer with whisk attachment. Beat on medium-high speed until stiff, glossy peaks form, and mixture is cool, about 10 minutes.
3. Switch to paddle attachment. With mixer on medium speed, add butter, 1 or 2 tablespoons at a time, mixing well after each addition. Mix, scraping down sides of bowl as needed, until mixture is smooth. Mix in vanilla extract. Use immediately, or refrigerate in an airtight container for up to three days.