In a medium bowl set over a pot of simmering water, combine the egg whites, sugar, and salt. Heat the mixture, whisking often, until it reaches 160 degrees and the sugar is dissolved.

Pour into a large bowl (if using a hand mixer), or the bowl of a standing mixer with the whisk attached. Whisk on medium high speed until stiff peaks form, about 8-10 minutes. The mixture should also be room temperature, at this point.

With the mixer on medium speed, add the butter, 1 piece at a time, and beat until incorporated before adding another piece. Continue to beat until thick and smooth (if it looks curdled or soupy, just keep beating it until smooth again, another 3 minutes or so). Stir in the vanilla until incorporate and tint with food coloring, as desired.