IN A SMALL BOWL, STIR TOGETHER CONFECTIONERS SUGAR AND CAKE FLOUR, SET ASIDE. MEASURE 1 CUP OF THE SUGAR/FLOUR MIXTURE AND SIFT TOGETHER WITH COCOA POWDER, SET ASIDE.
IN A LARGE BOWL, ADD EGG WHITES, CREAM OF TARTAR AND VANILLA. WITH MIXER ON HIGH SPEED, BEAT UNTIL SOFT PEAKS FORM. GRADUALLY ADD GRANULATED SUGAR, ABOUT 2 TABLESPOONS AT A TIME, BEATING UNTIL STIFF PEAKS FORM. TRANSFER 1/3 MIXTURE TO ANOTHER BOWL.
WITH A RUBBER SPATULA, FOLD IN PLAIN FLOUR MIXTURE, ABOUT 1/4 CUP AT A TIME, JUST UNTIL FLOUR DISAPPEARS. WITH RUBBER SPATULA, FOLD COCOA MIXTURE IN REMAINING EGG WHITE MIXTURE.
POUR HALF OF THE WHITE MIXTURE INTO UNGREASED 10 " TUBE PAN. GENTLY SPOON CHOCOLATE BATTER OVER WHITE BATTER IN PAN. TOP WITH REMAINING WHITE BATTER. MARBLE BATTER BY USING A OVER AND UNDER MOTION.
BAKE 40 TO 45 MINUTES OR UNTIL TOP OF CAKE SPRINGS BACK WHEN LIGHTLY TOUCHED WITH FINGER.
INVERT CAKE IN PAN ON A FUNNER, COOL COMPLETELY. USING A METAL NARROW SPATULA, LOOSEN SIDES OF CAKE FROM PAN, REMOVE CAKE TO SERVING PLATTER.