Preheat oven to 150C (302F).
Use a large bowl, pour in the milk. Place the bowl over simmering water. Don’t let the bottom of the bowl touch the water. Add cream cheese, stir occasionally, until completely dissolved and the mixture turns smooth. Stir in butter, till dissolved. Remove from heat. Let cool down a bit, then add the egg yolks and combine well. (Note: Make sure the mixture is not too hot, as you don’t want to cook the egg yolks at this stage.)
Combine cake flour and cornflour. Sift in the flours into the cream cheese mixture, a small amount at a time. Mix well between every addition, and make sure there aren’t any flour lumps. Stir in freshly grated zest. Set aside.
Place egg whites in a large clean bowl. (Note: Make sure there’s no oil or water in the bowl at all.) Use an electric mixer to beat the egg whites for 3 minutes, then add cream of tartar and blend again. Pour sugar in the egg whites and blend until very stiff peaks form.
Fold in the egg whites into the cream cheese mixture gently with a rubber spatula just until all ingredients are incorporated. Do not stir or beat. For a better result, fold in egg whites with a small amount at a time, at least 3 times.
Pour the mixture into the two baking pans. Place the pans into another larger baking tray. Add hot water in the tray up to half way. Bake for about 50 to 60 minutes. Test with a needle or skewer that comes out clean.
Turn off the oven. Leave the oven door ajar for 10 minutes. Remove from the oven and remove from the pans. Let cool completely on a wire rack. Chill in a fridge for about 3 hours. Enjoy!