Combine egg whites and sugar in a heatproof bowl. Set over a pan of gently simmering water.
Whisk constantly until sugar is dissolved and temperature of 160ºF is reached.
Transfer mixture to the bowl of a stand mixture with a whisk attachment. Whisk on low speed, gradually increasing to medium-high speed. Whisk until glossy, stiff peaks form and mixture reaches room temperature.
Turn mixer to medium-low speed. Add butter, a few pieces at a time, allowing the butter to fully incorporate each time before adding more.
Switch to paddle attachment. On low speed, mix in vanilla, salt and food color. Continue mixing on low speed for a few minutes until smooth.