In a bowl, combine the cake flour, baking powder, and sugar. Combine the egg and milk in a bowl and whisk together until well combined.
Pour the wet ingredients into the dry and whisk together until smooth. Transfer to a cup with a pour spout, cover with plastic wrap, and refrigerate for at least 30 minutes.
Heat a taiyaki pan over medium-low heat and spray with nonstick spray. Fill each mold about 50% full. Place your filling (about 1-2 tablespoons) of choice in the center of each mold. Pour additional batter over the filling, cover completely and make sure the batter is filled to the top.
Close the pan and flip. Cook for 3 minutes. Flip again and cook for another 3 minutes. Open and check that the Taiyaki is golden brown.
