In the bowl of a stand mixer, whisk together egg whites and sugar until frothy. Place bowl over a small saucepan of simmering water. Whisk mixture constantly until sugar dissolves and a candy thermometer registers 160°, about 2 minutes. Return bowl to mixer, and beat at high speed until stiff peaks form. Beat in vanilla. Add butter, 1 tablespoon at a time, beating well after each addition.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/frosting-finishing-touch/