Preheat the oil to 350°F in a deep fryer or medium saucepan.
In a medium bowl, whisk together flour, cornstarch, baking powder, salt, and cayenne pepper.
Add cold water and whisk until light and foamy.
Pat the fish dry with a paper towel.
Dip each piece into batter and let excess drip off.
Fry 2–3 pieces at a time for 3–4 minutes or until golden.
Drain on a wire rack before serving.
