Meanwhile, make Frosting: Place egg whites, sugar and cream of tartar in heatproof bowl of a stand mixer, then set bowl over saucepan of simmering water. Beat constantly with whisk about 5 minutes or until sugar is dissolved and mixture is warm to the touch. Remove from heat; transfer bowl to stand mixer. Using whisk attachment, begin to beat mixture on low speed. Gradually increase to high speed. Beat 5 to 7 minutes or until stiff peaks form. Stir in vanilla.