5 large egg whites, room temperature
1 cup plus 2 tablespoons sugar
Pinch of salt
1 lb. unsalted butter, at room temperature
2 teaspoons vanilla extract
To make the frosting, combine the egg whites, sugar and salt in a heatproof bowl set over a pot of simmering water. Heat, whisking frequently, until the mixture reaches 160°F/70°C and the sugar has dissolved.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, (about 8 minutes).
Reduce the speed to medium and add the butter, 2 tablespoons at a time, adding more once each addition has been incorporated. If the frosting looks soupy or curdled, continue to beat on medium-high speed until thick and smooth again, about 3-5 minutes more (don’t worry, it will come together!). Stir in the vanilla extract and mix. Tint with paste icing color if desired.