In a large bowl add olive oil and castor sugar. Whisk till well combined.
Then add 2 eggs, one by one and whisk till it becomes foamy. Now add vanilla extract and whisk again for a minute.
Sift in all the dry ingredients, wholewheat flour, all purpose flour, salt, baking soda, baking powder and cornstarch in a pan.
Add orange zest and mix until well combined.
Now add dry ingredients to wet ingredients in batches. Keep alternating between dry ingredients and orange juice. Add one batch of dry ingredients followed by little orange juice and then gently fold till combined. Keep repeating till all the dry ingredients are used up and the batter is well combined.
Pour the batter into a prepared pan. Tap on the counter 2 or 3 times to clear air pockets.