Preheat oven to 160C. Mix sifted plain flour and corn flour in a bowl.
Beat yolks and sugar with a hand whisk until slight pale. Add oil, orange juice and zest. Stir as you add.
Sieve the flour the second time into the yolk mixture. Stir as you add till smooth and egg mixtures into thick paste.
Beat egg whites until frothy. Add sugar in 3 separate rounds and whisk until stiff peak and glossy-looked.
Fold ⅓ of egg whites into the yolk mixture. Then pour in the remaining egg whites and fold gently until just combined with a rubber spatula.
Divide the batter into 2 equal portions, about 250g and mix one portion with 1 tablespoon of chocolate paste.
Scoop half portion of the plain batter into the 21 cm chiffon pan then 2nd layer with chocolate batter, remaining plain batter and end with chocolate batter. (**my chiffon cake is a little low, best to use a 20cm pan), lift and drop the pan gently to remove air bubbles.
Bake at 2nd lower rack in the oven for about 45 minutes.
Invert immediately and let to cool down completely on a wire rack.
Dust cake with cocoa powder as desire.
Notes
Chocolate paste - store bought from the bakery supply store
Recipe by Anncoo Journal at https://www.anncoojournal.com/orange-chocolate-chiffon-cake/