This recipe is used in our Lemon-Thyme Pound Wedding Cake with Meringue Buttercream.
Combine egg whites, sugar, and salt in a heatproof mixer bowl set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (mixture should feel completely smooth when rubbed between your fingertips).
Attach the bowl to a 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 mixture is fluffy and glossy and completely cool (test by touching bottom of 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 butter has been added, whisk in vanilla; add a drop or two of yellow food coloring, if using. Switch to the paddle attachment, and continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Scrape down the sides of the bowl with a rubber spatula and continue beating until frosting is completely smooth. Keep buttercream at room temperature if using the same day; or store in an airtight plastic container in the refrigerator for up to 1 week or in the freezer up to 1 month.