Pour vegetable oil into a heavy pot to a depth of about 3 inches (or use a deep fryer) and heat to 360°F (182°C).
In a large mixing bowl, whisk together the flour, cornstarch, sugar, salt, baking powder, baking soda, onion salt, paprika, and black pepper until evenly combined.
Slowly pour the cold club soda into the dry ingredients while stirring; the batter will foam—stir until smooth and combined.
Working with a few tenders at a time, dip each chicken tender into the batter to coat completely.
Carefully lower battered tenders into the hot oil and fry 2 to 3 minutes per side, until the coating is golden and the pieces float to the surface.
Use tongs or a slotted spoon to transfer the cooked tenders to a wire rack to drain and keep the coating crisp; do not cover.