Combine egg whites, sugar, and salt in the 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 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 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. If not, transfer to an airtight container and refrigerate up to 3 days or 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.

To tint buttercream, reserve some for toning down the color, if necessary. Add gel-paste food color, a drop at a time (or use the toothpick or skewer to add food color a dab at a time) to the remaining buttercream. You can use a single shade of food color or experiment by mixing two or more. Blend after each addition with the mixer (use a paddle attachment) or a flexible spatula, until desired shade is achieved. Avoid adding too much food color too soon, as the hue will intensify with continued stirring; if necessary, you can tone down the shade by mixing in some reserved untinted buttercream.