Grease and line 5 small round baking pans (diameter= 4-inch/10-cm) with parchment paper.
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).
Add in a tray of water (about 2 cups/500 ml) in the oven, preheat oven to 160°C/320°F.
Combine egg yolks, sugar and salt, beat until thick. Add in corn oil, beat until combined. Add in grated lemon zest and milk, mix until well blended. Sift together cake flour and baking powder, sift again into egg yolk mixture, mix until combined.
Add meringue to batter in 3 batches, mix gently using a balloon whisk.
Fold gently until well mixed using spatula, 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 5 mini baking pans equally, tap baking pans 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, until the top is golden brown.
Once remove from oven, drop baking pans onto countertop to minimise shrinkage. Unmould the cakes and set aside to cool completely on wire rack.