Heat a deep skillet or fryer filled with oil to a consistent temperature of 375°F.
In a spacious mixing bowl, combine the flour, cornstarch, baking powder, salt, sugar, baking soda, garlic powder, onion powder, and cayenne pepper if using. Whisk until fully blended and aerated.
Add the beaten egg to the dry ingredients, followed by the chilled club soda. Stir gently until a cohesive batter forms. It should be smooth with slight lumps; do not overmix.
One by one, immerse the dried fish fillets into the batter, ensuring they are evenly coated. Allow any extra batter to drip back into the bowl.
Carefully lower the coated fillets into the hot oil, frying them in batches. Cook each side for approximately 3 to 4 minutes or until a deep golden color develops and the coating is crisp.
Once fried, transfer the fillets to a wire rack set over a baking sheet to drain excess oil. Serve immediately while hot and crunchy.