Bring a few inches of water to a simmer in a medium saucepan over medium heat. Whisk the egg whites, sugar and salt in the bowl of a stand mixer. Set the bowl on top of the pan (do not let the bottom of the bowl touch the water). Cook, whisking, until the egg whites are frothy, the sugar dissolves and the mixture is warm, 3 to 5 minutes.
Transfer the bowl to the stand mixer and beat with the whisk attachment on medium-high speed until the mixture is cool and stiff glossy peaks form, about 5 minutes. Beat in the butter a few pieces at a time, waiting until they?re incorporated before adding more. The frosting may look liquidy or curdled but just keep beating until smooth and shiny. Add the vanilla and beat until fluffy, 3 to 5 more minutes.
Chocolate Swiss Buttercream: Make Swiss Buttercream, using just 1 teaspoon vanilla. After adding the vanilla, switch to the paddle attachment and beat in 8 ounces cooled melted semisweet chocolate until smooth and fluffy.