In a small mixing bowl set on a double boiler over medium heat, combine eggwhites and sugar. Whisk until sugar is fully dissolved and mixture appears foamy.
Transfer mixture to a Hobart Mixer fitted with a whisk attachment and add in kosher salt and vanilla extract. Whisk on high speed until stiff peaks form.
Gradually add in room temperature butter and continue to whisk on high speed for about 10- 15 minutes. Mixture may appear curdled during the process, but will come back together. Whisk until all butter has been added and buttercream is completely smooth and fluffy.
Transfer to an appropriate storage container and store refrigerated.
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