Preheat the oven: Preheat your oven to 150°C (300°F). Line an 8×4-inch loaf pan with parchment paper, making sure the paper overhangs on all sides. This will make it easier to remove the cake from the pan after baking.
Prepare the honey mixture: In a small bowl, mix the honey with the warm water until smooth. Set aside. This mixture will be added to the batter later to enhance the cake’s sweetness and moisture.
Whip the eggs and sugar: Place the eggs and granulated sugar in a large mixing bowl. Using an electric mixer, beat on high speed until the mixture becomes pale, thick, and increases in volume significantly. The batter should fall in long ribbons when you lift the whisk or beaters. This step is essential to achieving the light and airy texture of Castella.
Add the wet ingredients: Once the egg and sugar mixture has thickened, gently add the honey mixture, vegetable oil, and vanilla extract (if using). Mix on low speed until just combined. Be careful not to deflate the batter, as it should remain light and fluffy.
Sift in the dry ingredients: Sift the bread flour into the batter in two additions. Gently fold the flour in using a spatula, being careful to keep the batter airy. Folding slowly ensures that the batter remains light and the flour is incorporated without deflating the whipped eggs.
Prepare to bake: Pour the batter into the prepared loaf pan and tap the pan gently on the counter to release any large air bubbles that may have formed during mixing.
Set up the warm-water bath: Place the loaf pan inside a larger baking tray. Pour hot water (not boiling, just hot) into the tray, making sure the water comes halfway up the sides of the loaf pan. This creates a gentle, even heat during baking, resulting in a moist and smooth texture for the Castella cake.
