In a bowl, mix the all-purpose flour, cornstarch, sugar, and baking powder. Alternatively, you can use cake flour instead of all-purpose flour and cornstarch.
Gradually add the milk and whisk until smooth. If time allows, cover the batter and let it rest for one hour to improve texture.
If using red bean paste, measure 2 tbsp per cake and shape into rectangles for easy placement.
If using mozzarella cheese, slice into 2 cm thick sticks and set aside.
Heat the bungeo-ppang pan over low heat to ensure even cooking. Prepare a small bowl of vegetable oil, a pastry brush, and a spatula. Before cooking, whisk the batter again to maintain consistency. Brush the fish-shaped molds with vegetable oil and increase the heat to medium.
Pour the batter into the mold, filling it halfway. If the batter is thick, spread it evenly with a spatula.
Use a spatula to gently place one portion (2 tbsp) of red bean paste or a mozzarella cheese stick onto the center of each mold.
Pour additional batter on top until the filling is fully covered.
Cook for 2-3 minutes on one side.
Grease the other side of the mold, then flip the pan over and cook for another 2-3 minutes.
If the any side is not fully golden brown, cook for an additional minute.
Carefully remove the bungeo-ppang from the pan and let them cool slightly before serving. Enjoy them warm for the best taste!