In the stand mixer bowl, combine egg whites, 1 3/4 cups sugar, the cream of tartar and salt. Set bowl over a pan of simmering water. Whisk by hand constantly until mixture reaches 165°, about 4 minutes. (Start checking temperature after 3 minutes.) Lift out bowl, dry the bottom and snap into mixer. Whisk on high speed to glossy, cool, stiff peaks, about 4 minutes. Add vanilla and beat 1 minute more to return to stiff peaks. Transfer half of this meringue to a separate bowl; set aside.