Heat oven to 375°F.
Combine the dry ingredients. In a mixing bowl, combine the cake flour, cocoa, baking powder, and 1 cup of the granulated sugar. Stir or whisk to blend thoroughly.
Beat the egg whites. In another mixing bowl (not plastic), beat the egg whites with salt and cream of tartar until soft peaks form. Gradually add the remaining 1/2 cup of granulated sugar and continue beating to stiff peaks.
Make the batter. In a small bowl, whisk together the egg yolks, water, and vanilla; add to the flour mixture and beat just until blended. With a spatula, gently fold the egg yolk mixture into the stiff beaten egg white mixture.
Bake. Spoon the batter into an ungreased 10-inch tube cake pan and bake for about 32 to 38 minutes. Cool in the pan.
Glaze or dust. Remove from the pan and top with the glaze of your choice or dust with powdered sugar.