Taiwanese castella cake

This cake is an excellent canvas for any flavor experiments you want to do on a sponge cake.

What you'll need

Ingredients

• 80 grams cake flour
• 70 grams sugar
• 100 grams milk
• 6 eggs, separated
• 25 grams (2 tablespoons) unsalted butter
• 25 grams neutral oil
• 2 tsp vanilla extract

Equipment

• Electric hand mixer
• Whisk
• Rubber spatula
• Mixing bowls
• Small sauce pot
• Parchment paper (or cupcake cups if using a muffin tin)
• Loaf pan or 6" square pan or muffin tin
• Broiler pan or baking sheet (for water bath)

Instructions

1. Preheat the oven to 300ºF (150ºC). Don't use convection, else your cake will take forever to bake. Place a broiler pan or baking sheet filled with about quarter inch of water into the oven to heat.
2. Line the loaf pan with parchment paper. Add a bit of oil to the pan beforehand to help the parchment stick better to the pan.


3. Add milk, butter, and oil into a small sauce pot. Heat on medium until the butter has completely melted, but the milk hasn't started boiling (scalding).


Note: This photo is before heating
4. Stream the milk mixture into the egg yolks, whisking constantly. Whisk in the vanilla extract.


5. Sift the cake flour into the yolk mixture. Whisk everything together just until no dry flour remains, forming a thin batter.


6. Using an electric mixer, beat the egg whites until foaming (~20 seconds).


7. Slowly stream in the sugar into the eggs while continuing to beat. Mix until soft peaks form. (~3 minutes)


8. Add one third of the egg whites into the batter and whisk together until no white streaks remain.
9. Pour the lightened batter into the rest of the egg whites and fold together with a rubber spatula until no white spots remain.
10. Pour the batter into the prepared pan. Tap firmly on the counter top to even out the top and remove large air bubbles.
11. Place the pan into the middle of the water bath and bake for 40 to 45 minutes, or until a toothpick comes out clean and the top is a light golden brown.
12. Eat immediately, or place in the refrigerator to eat later (it spoils quickly!)


Variations

All other instructions remain the same unless otherwise specified.

Coffee flavor

Add 2T instant espresso powder to the milk mixture prior to heating. Reduce vanilla to ½ teaspoon, instead of 2 teaspoons.

Powdered-based flavors

Powders that I've tried are cocoa, matcha, or hojicha. Substitute 20 grams of the cake flour for your flavor of choice, and reduce the vanilla to ½ teaspoon. When sifting the cake flour, sift in the powder as well.

Black sesame

Reduce milk to 80 grams instead of 100 grams. Whisk 40 grams of black sesame paste (which can be found at Japanese grocery stores) into the egg yolk mixture.

Tea

For loose leaf tea, I've found the only way to get the tea flavor to come out is to soak it in milk in the refrigerator for 1 to 2 days beforehand. Stronger teas like earl grey can be done with tea bags instead.

Ingredient modifications

• 18 grams of loose leaf tea (e.g. jasmine, oolong, etc.) or 6 tea bags of strong tea (e.g. earl grey)

instruction modifications

1. Soak 10 grams of your desired loose leaf tea in the 100 grams of milk 1 to 2 days before the day you want to make the cake. The longer it soaks, the stronger the tea flavor.
2. For an even stronger flavor, also grind up 4-5 grams of the same tea in a spice grinder into a powder to add to the cake flour.
3. On the day of baking, pour the milk and tea leaves into the pot, and proceed with step 3. Let steep, covered, for 30 minutes before continuing on to step 4.
4. Before adding the milk mixture to the egg yolks, strain out the tea leaves.
5. All remaining instructions are the same.

Syrup-based flavors

You can make flavored syrups by cooking down equal parts juice (e.g. yuzu juice) and sugar until a thick syrup forms. I'll also count honey as a syrup in terms of consistency. To compensate for the sugar and liquid already present, I reduce the amount of sugar and milk used.

Ingredient modifications

• ½ teaspoon vanilla
• 20 grams sugar
• 80 grams syrup (e.g. honey)
• 70 grams milk

Instruction modifications

1. Warm the syrup for 10 seconds in the microwave to loosen up
2. After adding the milk mixture to the egg yolks in step 4, whisk in the syrup as well. Don't whisk syrups into the milk mixture by itself, as any amount of acidity in the syrup could cause the milk to curdle.
3. Proceed with the rest of the instructions as normal.

Gluten free

With gluten free flour mixes, you won't get the same coloration as with cake flour. Use the toothpick test to determine if your cake is done instead, and add on about 10-15 minutes to the normal bake time. The below lists out the ingredient modification for a vanilla cake, but you can apply any of the modifications above on top for flavor variation.

Ingredient modifications

• 80 grams of sugar
• 3 tsp vanilla extract
• 50 grams measure to measure flour
• 35 grams neutral oil