Wash and cook banana leaves then wipe dry and put it in a tin.
Put milk and butter in saucepan cook until 70°C.
Pour the butter milk into the bowl with all purpose flour and corn starch which is already shifted then stir well.
Mash bananas then pour it into the batter and stir well.
Separate the eggs yolks and whites. Put the egg yolks into the batter and stir well.
Add in few drops of lemon juice or vinegar and salt into egg whites and whipping it with high speed and add in sugar in portion (3 times), whisk until soft peak.
Pour boiling water in a baking tray and put it in the oven, preheat oven to 150°C.
Put 1/3 portion of egg whites into the batter and stir well for emulsification.
Pour the batter into the egg whites bowl, use a spatula to draw a zigzag to remove air bubbles in the batter until smooth.
Pour the batter from the top into the mould and bang the mould for few times to release the air bubbles.
Water bath 150°C baking, use bottom heat for first 25 minutes then top and bottom heat for 45 minutes.
When the cake is done must take it out and tear off the baking paper. Then cut it and enjoy it while it is still warm.
Putting vinegar/lemon juice in egg whites can help neutralize the alkalinity of the egg whites and stabilize the egg whites. The salt has the properties of coagulating protein and also helps to stabilize the egg whites.
The use of banana leaf is to enhance the aroma and baking paper effect, if you hard to find it, you can use baking paper instead.
Castella cake is a cake that must be unmoulded immediately after it is out from the oven. It is the most delicious stage to enjoy while it is still hot. If you can't finish it, put it in the refrigerator. Defrost the cake on the next day before eating it. It is recommended to finish it within three days. Otherwise, the cake will be getting dry.
When using bananas to make cakes, must use bottom heat for the first 25 minutes. The purpose is to make the cake solidify first, mainly to prevent the banana particles from sinking to the bottom due to gravity.
The baking mould must be seamless, otherwise water will enter the bottom of the mould when baking in a water bath.