How to Make It
Step 1
In an electric mixer bowl set over a saucepan of simmering water, whisk egg whites and sugar until sugar is nearly dissolved.
Step 2
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.)
Step 3
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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