Preheat oven to 325. Resift cake flour with cocoa powder, 3/4 c sugar and salt.
In large bowl, beat egg whites on low speed 1 1/2 minutes, or until frothy.
Sprinkle cream of tartar over whites. Beat on medium speed until white and foamy.
Slowly add remaining sugar, beating 2 to 3 minutes, or until soft (not stiff), smooth peaks form. Sprinkle one-third cocoa mixture and vanilla over egg whites.
Fold in with rubber spatula, keeping spatula under surface of batter as you fold.
Repeat two more times with remaining cocoa mixture. Do not overfold.
Pour into ungreased 9 or 10" tube pan. Bake in lower third of oven about 50 to 60 minutes, or until cake springs back when touched and wooden pick inserted in center comes out clean.
Remove from oven and invert tube pan on counter.
Let cool in pan about 2 hours before removing.