Prepare the batter: In a large mixing bowl, whisk together the flour, cornstarch, baking powder, salt, and sugar.
Add club soda: Gradually add the cold club soda to the dry ingredients, stirring gently until you achieve a smooth batter. Be careful not to overmix – a few lumps are okay.
Heat the oil: Heat oil in a deep fryer or a deep skillet to 350°F (175°C).
Prepare the fillets/tenders: Pat your fish fillets or chicken tenders dry with paper towels.
Coat the fillets/tenders: Dip each fillet or tender into the batter, allowing any excess to drip off.
Fry the fillets/tenders: Carefully place the battered fillets or tenders into the hot oil. Fry in batches, making sure not to overcrowd the fryer, until golden brown and cooked through, about 4-6 minutes depending on the thickness.
Drain and serve: Remove from oil and place on paper towels to drain. Serve hot with your favorite sides and sauces.
