Heat oil to 350°F in a deep fryer or a medium-sized saucepan.
Make sure it's hot enough to crisp up the fish nicely.
In a mixing bowl, combine the flour, cornstarch, baking powder, salt, and cayenne pepper.
Stir until well mixed.
Pour the cold beer into the dry ingredients slowly.
Whisk until you have a smooth batter, and it's key to keep the beer cold for a light batter.
Dip each fish fillet into the batter, ensuring it is completely coated.
Lift and let any excess batter drip off.
Carefully place the coated fish into the hot oil.
Fry them in batches until golden brown, usually about 4-5 minutes per side.