Instructions:
1. Preheat the Oven:
- Preheat your oven to 325°F (163°C). Line a rectangular cake pan (typically 8×8 inches) with parchment paper, leaving some overhang for easy removal later.
2. Sift the Cake Flour:
- Sift the cake flour to ensure it is light and free of lumps. This step is crucial for achieving the desired airy texture in the Castella cake.
3. Separate Eggs:
- Carefully separate the egg yolks from the whites. Place the yolks in one bowl and the whites in another. Make sure there is no trace of yolk in the egg whites, as any fat can hinder the process of whipping them into stiff peaks.
4. Whip Egg Whites:
- Using a hand or stand mixer, whip the egg whites on medium-high speed until soft peaks form. Gradually add half of the sugar and continue whipping until stiff peaks form. The whites should be glossy and hold their shape.
5. Beat Egg Yolks:
- In a separate bowl, beat the egg yolks and the remaining sugar until the mixture becomes pale and thick. This will take a few minutes.
6. Combine Egg Yolks and Whites:
- Gently fold the egg yolk mixture into the whipped egg whites until well combined. Be careful not to deflate the whites, as they provide the cake’s lightness.
7. Add Flour Mixture:
- Sift the cake flour into the egg mixture in three parts, folding gently after each addition. Make sure there are no streaks of flour, but be cautious not to overmix.
8. Warm Honey, Mirin, and Water:
- In a small saucepan, warm the honey, mirin, and water over low heat until well combined. This mixture adds moisture and a subtle sweetness to the Castella cake.