In a large mixing bowl, whisk together the flour, cornstarch, baking soda, baking powder, salt, sugar, onion powder, and garlic powder until fully combined.

Slowly whisk in the chilled club soda until the batter achieves a smooth, pourable consistency similar to heavy cream.

Allow the batter to rest at room temperature for 10 minutes to hydrate the flour and activate the leavening agents.

Preheat frying oil in a deep pot or fryer to 375°F (190°C).

Dredge the protein of choice (fish fillets or chicken) in a thin layer of plain flour before dipping into the batter to ensure proper adhesion.

Carefully submerge the battered protein into the hot oil, frying for 3 to 5 minutes until the exterior is golden brown and reaches a crisp texture.

Remove the items with a slotted spoon and drain on a wire cooling rack to prevent the bottom from becoming soggy.