Preheat your oven to 325°F (160°C). Grease a rectangular cake pan with vegetable oil and line it with parchment paper.

In a large mixing bowl, beat the eggs and sugar together until the mixture becomes thick and pale. This may take about 15 minutes, so be patient.

Sift the cake flour, baking powder, and salt together into a separate bowl. This helps to aerate the dry ingredients and ensures an even distribution.

Add a portion of the dry mixture to the egg mixture. Using a spatula to fold them. Avoid stirring vigorously to maintain the airiness of the batter. Repeat the process until all the dry ingredients are incorporated, and the batter is smooth and well-mixed. Do not overmix.

In a separate bowl, mix the honey and warm water together until well combined. Gradually add this mixture to the batter, continuing to fold gently.

Bake in the preheated oven for about 40-45 minutes or until a toothpick inserted into the center comes out clean.

Once baked, let the Castella in the oven for a few minutes with the oven door opened. Take the cake out of the tin, flip it upside down, and peel off the parchment paper.

Wrap the cake in plastic film to keep it moist. Let it cool completely, then pop it in the fridge overnight. This step is crucial for the Castella to develop its characteristic moist and fluffy texture.