Sift sugar, flour, and cocoa together; set aside.
Beat egg whites (at room temperature) at high speed of an electric mixer until foamy.
Add cream of tartar and vanilla; beat 5 minutes or until stiff peaks form.
Sprinkle one-fourth of flour mixture over egg whites; gently fold in with a rubber spatula.
Repeat procedure with remaining flour mixture, adding one-fourth of the mixture at a time.
Spoon into an un-greased 10-inch tube pan with removable bottom.
Bake at 375° for 40 minutes or until cake springs back when touched lightly with fingers.
Invert pan on funnel or bottle for 2 hours or until cake is completely cooled.
Loosen cake from sides of tube pan, using a small metal spatula; gently remove from pan.