In a large bowl whisk together the flour, baking powder, and beer until smooth thin batter forms. It should be thinner than a pancake batter but slightly thicker than a crepe batter. Loosely cover with plastic wrap and let stand at room temperature for at least 30 minutes.
Cut the cod fillets into (6-inch) strips. Pat the fish with a paper towel to make sure it is completely dry.
Mix together in a small bowl, the salt, pepper, garlic powder and cayenne.
Season both sides of the cod with some of the seasoning mix. Reserve half of the seasonings for the dredging flour.
In a shallow dish (a pie plate works perfectly), pour the 1 cup of all-purpose flour and the reserved seasoning mix, whisk or stir to combine. (This step makes the beer batter adhere to the fish.)
In a large pot, dutch oven or deep fryer, heat oil to 375°. Fill up the pot halfway with oil. Once the oil is heated to 375 degrees, you are ready to fry the fish.
Working with one strip at a time, dip the fish into the seasoned flour, coat evenly on both sides. Then dip the fish into the beer batter, completely dunking it until it is fully submerged. Drain excess batter and then carefully drop the battered fish into the oil. Don’t overcrowd the pot, work in batches. Fry for 4 to 6 minutes on the first side. Flip it over and continue to cook for another 2 to 3 minutes on the second side. Only flip the fish once. Remove from the oil and drain on paper towels. Transfer to a cooling rack placed on top of a baking sheet. Repeat with remaining fish. (To keep the fish warm and crispy until ready to eat, place the baking sheet in a 200°F oven.)
Serve warm with lemon wedges, malt vinegar, and tartar sauce.