Double the quantities if you have more pieces of fish.
Make sure the cod pieces do not contain excess water. Pat them dry with a kitchen towel. Sprinkle salt on each piece of cod.
Place a large food bag and weigh the flour into it. Place the bag in the freezer for 15 minutes.
Heat the oil to 180°C in a deep saucepan or a deep fryer.
Place the cold flour, baking powder and 1/2 teaspoon of salt in a bowl. Gently mix with a whisk or a wooden spoon until combined.
Pour the cold beer over the flour, baking powder and salt mixture, and whisk for 1–2 minutes until you get a smooth batter (without lumps). Use the batter immediately: dip the cod fillets into it, shake off any excess, then carefully place them in hot oil (≈180°C) and fry for 8–10 minutes, until golden and cooked through, turning once halfway. Remove with a slotted spoon and drain on paper towels.
Serve with French fries, and add a squeeze of lemon over the fish for extra freshness.