In a large mixing bowl, sift the cake flour, sugar, baking powder, baking soda, and salt. Whisk to combine and set aside.

In a medium mixing bowl, add the egg, milk, vegetable oil, and vanilla extract. Mix to combine.

Pour the wet ingredients into the dry ingredient mixture. Whisk until just combined leaving a few clumps. Cover and rest in the refrigerator for at least 1 hour.

Preheat the taiyaki pan over medium low heat. Brush both sides of the mold with neutral oil and gently wipe away excess with a paper towel.

Give your batter one quick mix and transfer the batter into a measuring cup with a spout. Then pour the batter into the taiyaki mold about 60% full leaving the fishtail empty.

Add about 1 tablespoon of filling in the center of the fish. For easy filling, place the filling in a piping bag and pipe the filling into the batter.

Cover the filling with batter until it fills the fishtail. Close the mold and immediately flip so the batter spreads evenly.

Cook for about 2-3 minutes on each side until golden brown. Then transfer to a rack to cool slightly before serving. Cut any excess batter with scissors. Repeat with the remaining batter. Enjoy immediately when warm.