Set oven at 400°F. Separate eggs; beat egg yolks 1 minute. Add pinch of salt to egg whites; beat until foamy. Add cream of tartar and continue beating whites until they cling to bottom and sides of bowl, then beat 1 minute longer. Fold beaten egg yolks into egg whites gently but quickly. Fold in sifted sugar, flour, cocoa and vanilla. Do not over mix. Pour batter into prepared pan, spreading evenly from center out with a spoon. Strike pan several times on table to break any air bubbles. Bake in center of oven 18 to 20 minutes or until cupcake springs back when touched lightly. Cool 10 minutes in pan on rack; remove and cool completely before decorating.