Preheat the oven to 180 °C and keep a chiffon cake tin with steel pins at hand, but do not grease.
Blend the cocoa and boiling water to form a smooth paste.
Add the vanilla essence.
Beat the egg whites until frothy and add the cream of tartar.
Continue beating until stiff peaks form.
Add the castor sugar by the spoonful, beating continually until all the sugar has been added and the mixture is thick and shiny.
Add about 250 ml of the egg white mixture to the cocoa mixture and mix lightly.
Sift the cake flour and salt over the remaining egg white mixture and fold in gently.
Fold in the cocoa mixture.
Carefully turn the batter into the ungreased tin and gently knock the bottom of the pan on the work surface to remove any air bubbles.
Bake for 40 minutes or until done and a testing skewer comes out clean.
Remove from the oven and turn the tin upside down on the work surface.
Leave the cake to cool.
When cool, loosen the edges and carefully remove the cake.
Dust with cocoa powder and castor sugar when cool.
Makes 1 medium-sized cake.
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