1 cup plus 2 tbl sugar
pinch of salt
1 pound (4 sticks) unsalted butter, cut into tablespoons, room temperature
1 1/2 tsp pure vanilla extract
Combine egg whites, sugar and salt in a heatproof bowl of a standing mixer set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips.)
Attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom side of the bowl), about 10 minutes.
With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition. Once all the butter has been added, whisk in vanilla. Switch to the paddle attachment and continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Scrape down sides of bowl with a flexible spatula, and continue beating until the frosting is completely smooth. Keep buttercream at room temperature if using the same day, or transfer to an air tight container and refrigerate up to 3 days for freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.