Coat 9-inch/23-cm round baking pan with oil, then line with parchment paper.
Add in a tray of water, about 2 cups in the oven (to increase the humidity which helps cake bake evenly), then preheat oven to 160°C/320°F.
Whisk egg whites over low speed until large bubbles are formed. Add in cream of tartar, whisk until small bubbles are formed. Add in sugar gradually, whisk until stiff peak (the peaks will point straight up and hold), do not over whip.
Combine egg yolks, salt and sugar, beat until thick. Add in corn oil, beat until combined. Add in milk, mix well.
Sift together cake flour, cocoa powder, baking powder and baking soda, then sift again into egg yolk mixture, mix until just combined.
Add meringue to batter in 3 batches, mix gently using a balloon whisk. Fold gently with spatula until well mixed, then tap mixing bowl on countertop for a few times to burst large bubbles. (Reminder: Do it quick but gentle to avoid deflating the meringue. Scrape the bottom of mixing bowl with spatula and fold over, repeat folding motion until just well mixed.)
Pour batter into baking pan, tap baking pan again for a few times.
Bake at 160°C/320°F for 25 minutes, then increase to 180°C/355°F and bake for another 15 minutes, or until fully cooked (wooden stick inserted in the center of the cake comes out clean).
Once remove from oven, drop the baking pan from 1 foot high onto countertop (to minimize shrinkage).
Unmould Chocolate Sponge Cake and set aside on wire rack until completely cooled.