In a deep skillet or fryer, heat oil to 350°F (175°C).
In a large bowl, whisk together flour, cornstarch, baking powder, sugar, baking soda, salt, onion powder, and garlic powder.
Slowly whisk in the cold club soda until smooth. The batter should be slightly thick but pourable.
Pat fish or chicken dry with paper towels. Lightly dust with flour (optional—helps batter stick).
Dip each piece into the batter, letting excess drip off.
Fry in hot oil for 3–5 minutes per side, until golden brown and crispy.
Remove and place on a wire rack or paper towels to drain. Serve hot with tartar sauce or malt vinegar.