Steamed version: Bring the water of steamer to the boil.
Baked version: Add a tray of water (about 2 cups) in the oven, preheat oven to 200°C/395°F.
Grate orange zest from 3 oranges (orange part only).
Squeeze out orange juice from fresh oranges (strain through), measure out 75 g of orange juice.
Beat eggs, add in salt and sugar, beat until combined.
Combine 1 Liter of plain water with 400 ml of hot water (from electric thermo pot) in a large steel bowl to yield warm water with temperature about 45°C/113°F.
Dip mixing bowl in warm water bath, beat egg mixture over medium-high speed for 5 minutes until stiff. (Reminder: Beating egg mixture over warm water will shorten the time for it to become stiff. If it is not stiff enough, the batter will deflate quickly.)
Combine plain flour and cornstarch, sift through and mix well.
Add flour and orange juice alternatively into egg mixture in 3 batches, fold gently with spatula until well mixed. Add in corn oil, fold until well mixed. Add in grated orange zest, fold until well mixed.
Pour batter equally into 6 lined baking cups (7.5-cm).
Steamed version: Arrange baking cups in steamer rack, steam over medium heat for 8-9 minutes.
Baked version: Arrange baking cups on wire rack, bake in the preheated oven at 200°C/395°F for 25 minutes, or until golden brown. (Reminder: Different oven functions differently, baking time is just for reference, you may adjust the baking time if necessary.)
Unmould the steamed/baked egg sponge cakes and cool on wire rack.