In medium glass or metal bowl, stir egg whites, sugar, cream of tartar and vanilla with whisk until blended. Set over pan of simmering water, stirring occasionally with whisk, 16 to 20 minutes or until sugar has dissolved and egg mixture reaches 160°F. Pour into large bowl; beat with electric mixer on high speed 8 to 10 minutes or until stiff glossy peaks form. Use small icing spatula to swirl meringue over top of ice cream layer. Freeze at least 3 hours or overnight.