Peel and cut potatoes into fry-shaped slices. Soak in cold water for 30 minutes. Drain and pat completely dry.
Heat oil to 325°F. Fry potatoes for a few minutes until softened but not browned. Remove from oil and let rest. Increase oil temperature to 375°F, then fry again until golden and crispy. Season with salt.
Pat cod dry and season both sides with salt and black pepper.
In a shallow plate, add flour for dredging.
In a bowl, whisk together flour, cornstarch, baking powder, paprika, garlic powder, salt, and black pepper. Slowly pour in cold beer and whisk until smooth.
Coat the cod in flour first, then dip it into the beer batter. Let excess batter drip off.
Carefully deep fry the fish until golden brown and crispy. This takes around 4-6 minutes.
To make the tartar sauce, combine mayonnaise, chopped pickles, lemon juice, Dijon mustard, and salt. Mix well.
Serve hot with crispy fries and tartar sauce.