In a bowl, whisk flour, baking powder, and salt.
In another bowl, beat egg with sugar. Add milk and stir until smooth.
Combine wet and dry ingredients. Mix until the batter is like pancake batter.
Heat a Taiyaki pan on medium heat and lightly grease it.
Pour a thin layer of batter into one side of the fish mold.
Add about 1 teaspoon of red bean paste in the center.
Cover with more batter until the mold is filled.
Close the pan, cook 2–3 minutes, then flip and cook another 2–3 minutes.
Check for a golden-brown color. Remove carefully.
Serve warm and enjoy.