Let’s dive into the preparation. Following these steps will take you through the delightful process of baking your Taiwanese Honey Castella sponge cake.

First, preheat your oven to 160°C (320°F). Ensuring your oven is at the right temperature is crucial for that perfect bake.

In a small saucepan, melt the unsalted butter over low heat. Once melted, allow it to cool slightly. This will help prevent the eggs from cooking when combined later.

In a mixing bowl, whisk together the egg yolks, honey, granulated sugar, and vanilla extract until the mixture is pale and slightly thickened. This step incorporates air, which is critical for a light, airy cake.

Gradually whisk in the whole milk until it's well blended. Sift the all-purpose flour into the mixture and gently fold it in. Be careful not to over-mix; you want to maintain all that glorious air you just incorporated.

Slowly fold in the melted butter, ensuring it's well combined without deflating your batter.

In a clean bowl, beat the egg whites until stiff peaks form. This is key for achieving that spongy texture. When you can turn the bowl upside down without the egg whites moving, you’re in business!

Gently fold the whipped egg whites into the butter and yolk mixture. Take your time with this step; the goal is to retain the volume of the egg whites while evenly distributing everything.

Line a square or rectangular cake pan with parchment paper. Carefully pour the batter into the prepared pan. Tap the pan lightly on the counter to release any air bubbles.

Bake the cake in the preheated oven for approximately 40-50 minutes or until a skewer inserted into the center comes out clean. The top should be a lovely golden brown.

Once baked, let the cake cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely. Slice it only when it has cooled for the best texture.