In clean bowl with clean whip, whip egg whites and salt until foamy. Add cream of tartar and continue to beat until stiff but not dry. Fold 1/3 egg whites into yolks until just blended; gently add remaining egg whites and flour mixture. Add vanilla extract. Gently fold together just until no streaks remain. Divide batter evenly into cavities, spreading to edges. Cavities should be about 1/2 full.