In a mixing bowl, whisk together the flour, baking powder, and cornstarch.
In another bowl, whisk the egg and sugar until combined. Add milk and vanilla extract.
Gradually pour the wet mixture into the dry, whisking until smooth.
Heat your taiyaki pan over medium heat and lightly grease it with oil.
Pour batter into one side of the mold, filling about 60% of the fish shape.
Add a spoonful of red bean paste (or other filling) in the center.
Cover with a bit more batter to seal the filling.
Close the pan and cook each side for 2–3 minutes, flipping until golden brown.
Remove and let cool slightly before serving.