In a large bowl, sift together cake flour, baking powder, and salt. Add egg yolks, corn oil, ¾ cup water, calamansi juice, and 1/2 cup sugar then mix the ingredients well.
Combine egg whites and cream of tartar in another bowl then beat them by hand or with an electric mixer fitted with the whisk attachment until the egg mixture is frothy.
Gradually add the remaining sugar and beat the egg mix until it is stiff but not too dry. This should take about 7 to 8 minutes.
Gently fold the egg yolk into the egg-white mixture using a large rubber scraper. You'll want to be careful if you want to avoid deflating the egg whites.
Pour the batter into the pans over the cooled custard.
Bake for 45 minutes or until a toothpick inserted in the center comes out clean with a few moist crumbs adhering to it.
Cool the cake for around ten minutes then run a knife gently around the pans and invert the cakes onto serving platters.
You can serve the cake cold or at room temperature.