In a large bowl, sift together the All-Purpose Flour, Baking Powder, Sugar and Salt so the dry ingredients are evenly combined.
In a separate bowl, whisk the Egg with the Milk until smooth and slightly frothy.
Pour the wet mixture into the dry ingredients and stir gently until just combined — a few small lumps are fine. Let the batter rest for 15–30 minutes to allow the flour to hydrate (this gives a better texture).
Preheat your taiyaki pan or fish-shaped mold over medium-low heat and lightly brush or wipe the cavities with Neutral Oil to prevent sticking.
Spoon a thin layer of batter into each mold cavity so the bottom is covered (about one third full). This creates the base shell.
Place about 1 heaping tablespoon of Tsubu-An (sweet chunky red bean paste) in the center of the batter, keeping it away from the edges so it won’t leak.
Top the filling with a little more batter to cover (another one third full), then close the mold. If your pan is open-style, pour batter into both sides, add filling, and close the two sides together.
Cook for 2–3 minutes on one side, then flip the mold (or turn the pan) and cook another 2–3 minutes, adjusting the heat as needed so the taiyaki become golden brown and crisp without burning.
Open the mold and carefully remove the taiyaki. Let them cool 1–2 minutes so the filling firms slightly, then serve warm for the best texture and flavor.
