How should I convert my Victoria sponge to a chocolate sponge? I generally bake Victoria sponge cake with 6 oz flour. Now to make a chocolate sponge cake do I substitute 2 oz of flour with cocoa powder or do I keep the 6 oz flour and add additional 2 oz cocoa powder. 

I have seen some recipes that mention the former and some recipes mention the latter of what I mentioned above. Which of these 2 methods is the best way to prepare a chocolate sponge cake.
Replace 2oz of flour with the cocoa powder. If you add an extra 2oz of dry ingredients, the ratio of wet to dry will be off and you will end up with a dry sponge.