When cheesecakes are chilled completely, remove cheesecakes from the pan carefully, removing the bottom disc. Begin preparing frosting. Add egg whites, sugar and cream of tartar to a metal stand mixer bowl over simmering water. Whisk for several minutes or until sugar is dissolved and whites are warm. Move metal bowl to mixer, beat on low increasing to high until mixture is stiff and glossy, roughly 5 minutes. Add vanilla, mixing in well. Transfer to a piping bag
with desired tip, pipe onto chilled cheesecakes.