An incredibly soft, smooth velvety chocolate castella cake, perfectly light with a springy texture are some of the characteristics of Taiwan castella cake that will leave you craving for more.
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 and separate the whites from the yolks into separate mixing bowl. They are easier to separate when cold. Let them come to a room temperature.
We will bake the cake in a water bath. Get another larger pan and fill it up with water, about 1/2 inch full and put it inside the oven, on the middle rack. So the water will heat up together with the oven
I use 7-inch round pan with a loose-bottom piece. Line your cake pan with parchment paper on the bottom and the sides. If you use a loose-bottom pan like me, put the pan inside another pan. I found that water still manages to sip in when I double wrapping with aluminum foil
Prepare the cake batter:
Combine the cake flour, cocoa powder, and salt. Set aside
Set up a double boiler (bain marie):
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
Add butter and milk to the double boiler you set up and whisk until the butter melts. Add the chocolate and whisk until melt. Do not overheat. Remove from the heat and wipe the bottom of the bowl
Add the yolks and whisk to combine. Whisk in the flour mixture and use a whisk to combine again
Prepare the egg white meringue:
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
Fold the meringue into the cake batter:
Add 1/3 of the meringue into the cake batter. Use a whisk to swipe down and fold over to combine and then continue with the next 1/3 of meringue. After adding the last 1/3 of meringue, switch to spatula 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. Decorate the top very gently with some mini chocolate chips
Bake the cake:
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
Cooling down:
Remove from the oven and immediately and bang on the countertop a few times to minimize shrinkage. Then push the cake out of the pan. 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
Serve:
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