Taiwanese Pandan Castella Cotton/Sponge Cake

Preheat oven to 300 F (150 C) for conventional oven. If you use convection oven, lower the temperature by 20 degrees. Get the eggs out from the fridge for about 15 minutes, to let them come to room temperature

Line your cake pan with parchment paper on the bottom and the sides. I used two parchment paper overlapping. So I can lift the cake out from the pan for cooling down later. If you use loose-bottom pan, double wrap the outside with aluminum foil to prevent water from sipping into the cake during baking

We will bake the cake in a water bath. Get another larger pan so you can put the cake pan in for a water bath. Boil some water for the water bath

Put some water in a saucepan and set a heat-proof bowl sitting on top of the saucepan. The water shouldn't be touching the bottom of the bowl. Pour milk and oil and heat until the mixture is warm, but not boiling. Remove from the heat and wipe the bottom of the bowl

Sift in cake flour and salt into the milk mixture. It will be a thick mixture. Add the pandan extract and then add the egg yolk one at a time. Set aside

Beat egg whites on low speed until foamy, then add vinegar and continue beating for another minute and then increase the speed to medium (speed 6 on KA) and add sugar in 3-4 batches and continue to beat until you get a medium-soft peak. When you lift the meringue up with a whisk, the tail will bent. Do not overwhip the meringue

Add 1/3 of the meringue into the cake batter. Swipe down and fold over to combine and then pour the batter into the meringue and then swipe down and fold over to combine until you no longer see the white meringue. Try to work fast and not to overfold the meringue and deflate it

Pour the batter into the prepared pan. Bang the cake pan on the countertop a few times to pop large air bubbles. Place the cake pan inside the larger pan. Pour in hot water about 1 inch height

Very carefully transfer the cake pan and put it inside the larger pan with water that you prepared earlier. Bake for the next 1 hour 50 minutes to 2 hours. Took me 2 hours with my oven. You can start checking on the cake at about 1 hour 30 minutes mark. The cake should bounce back when you gently touch it. It should be dry to touch. A cake tester should come out clean with maybe a few crumbs but it shouldn't be wet

Remove from the oven and immediately and bang on the countertop a few times to minimize shrinkage. Then immediately lift the cake out from the cake pan very gently by grabbing on the parchment paper. Place on a cooling rack and gently peel off the parchment paper on the side of the cake and let it cools down completely this way before cutting. The cake will jiggle when you pat on it. It will stop jiggling once the cake has cooled down completely

Use a serrated knife to cut the cake so you won't squish down the cotton soft cake. The castella cake can be kept in room temperature for 3 days in an air-tight container