Combine egg whites, and sugar in the heatproof bowl of an electric mixer. Set bowl over a pan of simmering water. Whisk constantly until sugar has dissolved and whites are hot to the touch, 5 to 7 minutes.
Transfer bowl to the electric mixer. Using the whisk attachment, mix on low speed, gradually increasing to high speed, until stiff, glossy peaks form, about 10 minutes.
At low speed, add butter to egg whites, and beat until smooth. Add vanilla and salt, and continue mixing until incorporated.