Prepare the fish by cutting the fillets into serving-size pieces, around 4 inches each.
Pat them dry to ensure the batter sticks well.
In a mixing bowl, combine flour, baking powder, salt, and cornstarch.
Mix in the egg and water, whisking until you have a smooth batter.
Heat vegetable oil in a deep fryer or heavy-bottom pot to 350°F.
Make sure the oil is hot enough to properly fry the fish.
Dip each fish piece into the batter, ensuring it is fully coated.
Allow excess batter to drip off before placing the fish in the oil.
Fry the fish in small batches for about 3 to 4 minutes per side, or until the batter is golden and crispy.
Remove and drain on a paper towel.