Bring 1 inch of water to a simmer in a large skillet. Combine cream of tartar and 2 teaspoons water in 4- to 6-cup stainless-steel bowl. Whisk in whites and sugar.
Place thermometer near stove in a mug of very hot tap water. Set bowl in skillet. Stir mixture constantly with rubber spatula, scraping bottom and sides often to avoid scrambling whites. After 45 seconds, remove bowl. Quickly insert thermometer, tilting bowl to cover stem by at least 2 inches. If less than 160°F, rinse thermometer in skillet water and return it to mug. Replace bowl in skillet. Stir until temperature reaches 160°F when bowl is removed. Beat on high until whites are cool and stiff. Set aside.
Whip heavy cream with vanilla until it holds soft peaks but is not too stiff. Fold egg white mixture into cream. Cover and chill. Refrigerate up to 8 hours.