In an electric mixer bowl set over a saucepan of simmering water, whisk egg whites and sugar until sugar is nearly dissolved.
Remove bowl from pan. Whip mixture at high speed until stiff. Reduce speed and beat until cool, 5 minutes. (Mixture must be cool before adding butter.)
Add cold butter 1 piece at a time at mediumlow speed. Beat at high speed until fluffy. Add salt and vanilla and mix at low speed until well blended.
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