Line 9-inch/23-cm round baking pan (no joining part at the bottom) with non-stick baking paper. (Reminder: If you use mould with joining parts at the bottom, line the outside of the baking pan with a thick layer of paper kitchen towels and wrap with 2 layers of aluminium foil. If it is without joining parts, no need to wrap with foil, just put a piece of cloth at the bottom of the baking pan with batter to replace the kitchen towels.)
Grate orange zest from 4 oranges (orange part only).
Squeeze orange juice from fresh oranges (strain through), measure out 110 g of orange juice.
Preheat oven to 150°C/300°F. (Reminder: Different oven works differently, baking temperature is just for reference, adjust the temperature according to your own oven if necessary.)
Combine orange zest, orange juice and corn oil, mix well. Sift in cake flour, mix until combined. Combine egg and egg yolks, add to batter gradually, mix until well combined.
Whisk egg whites on low speed until foamy. Add in cream of tartar and salt, whisk until tiny bubbles are formed. Add in sugar gradually, whisk on low speed until soft peak (meringue is shiny with arch shape). (Reminder: Egg whites and equipment used to beat egg whites must be grease-free, water-free and egg yolk-free. It is advisable to clean the egg shells and equipment with dishwasher detergent, then dry with a clean cloth. Use a clean small bowl to separate the egg yolk and egg white, if the egg yolk cracks, don’t use that egg white.)
Add meringue to egg yolk batter in 4 batches, mix gently with hand whisk.
Fold gently with spatula until well mixed. Tap mixing bowl a few times on countertop to burst large bubbles.
Pour the batter into prepared baking pan, tap baking pan on countertop a few times.
Fill a 10-inch square baking pan (line with a piece of kitchen cloth) with hot water (about 1-inch high, for water bath), put in baking pan with batter.
Bake in the preheated oven at 150°C/300°F for 1 hour 50 minutes, until fully cooked. (Reminder: As a guideline, if your cake shrinks a lot when cooled, then the oven temperature is too hot, reduce the baking temperature. The cake should rise slowly during baking to prevent it from over-rising and then collapsing when out of oven.)
Drop the baking pan from 1 foot high onto countertop after removing from oven to minimise shrinkage.
Unmould the cake and cool completely on wire rack (at least 6″ from countertop).