Preparation In large metal bowl, whisk together 1/3 cup water, sugar, egg whites, cream of tartar, and salt. Set bowl over pan of barely simmering water and mix with handheld electric mixer at low speed. Gradually increase speed to high, beating until mixture holds stiff peaks, about 5 minutes. Transfer bowl from pan to folded kitchen towel on counter and continue beating until mixture is cool and billowy, about 2 minutes more. Beat in vanilla. (Frosting can be made 4 hours ahead and chilled, covered.)
Preparation In large metal bowl, whisk together 1/3 cup water, sugar, egg whites, cream of tartar, and salt. Set bowl over pan of barely simmering water and mix with handheld electric mixer at low speed. Gradually increase speed to high, beating until mixture holds stiff peaks, about 5 minutes. Transfer bowl from pan to folded kitchen towel on counter and continue beating until mixture is cool and billowy, about 2 minutes more. Beat in vanilla. (Frosting can be made 4 hours ahead and chilled, covered.)