Preheat the oven to 375 degrees. Have an ungreased 10-inch tube pan with a removable bottom ready.
Sift the sugar, flour and cocoa together; set aside.
Beat the egg whites at high speed until foamy. Add the cream of tartar and vanilla; beat 5 minutes or until stiff peaks form. Sprinkle one-fourth of the flour mixture over the egg whites; gently fold in with a rubber spatula. Repeat procedure with the remaining flour mixture, adding one-fourth of the mixture at a time.
Spoon into the ungreased tube pan and bake for 40 minutes or until the cake springs back when touched lightly with the fingers. Invert the pan on a funnel or bottle for 2 hours or until the cake is completely cooled.
Loosen the cake from the sides of the tube pan, using a thin-bladed knife or metal spatula. Gently remove the cake from the pan.