Use this with the Cupcake Ice-Cream Cones recipe.
Combine egg whites, sugar, and salt in a heatproof mixer bowl, and set over a pan of simmering water. Whisk constantly by hand until the mixture is warm to the touch and sugar has completely dissolved (test by rubbing mixture between your fingertips; it should be smooth).
Transfer the bowl to the mixer stand, and whisk on low speed. Gradually increase speed to medium-high, and whisk until fluffy and cooled. Continue whisking until stiff, glossy peaks form, about 10 minutes.
Reduce speed to medium-low, and add the butter a few tablespoons at a time, blending well after each addition. Whisk in the 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. To tint buttercream, add a small amount of food coloring at a time. Keep buttercream at room temperature if using the same day.