2 cup(s) all-purpose flour
1 teaspoon(s) baking powder
1/2 teaspoon(s) salt, plus more for serving
1 teaspoon(s) malt vinegar, plus more for serving
1 gallon(s) canola oil
2 pound(s) russet potatoes, preferably Russet Burbank
4 (6-ounce) cod fillets
Combine flour, baking powder, and salt in a mixing bowl. Whisk in 1 3/4 cups water and stir in vinegar; set batter aside.
Heat canola oil to 300 degrees; in a deep fryer or a deep, heavy pot with a candy thermometer. Peel and cut potatoes lengthwise to form long, 1/4-inch-thick "chips." Add to oil, and fry 10 minutes, stirring occasionally. Remove and drain well on paper towels, then place in refrigerator to chill, 10 to 15 minutes. Keep oil in pot on stove, but turn off heat.
Bring oil to 350 degrees. Fry chips a second time, in two batches, until golden brown and crisp, 2 to 3 minutes per batch. Drain well on clean paper towels. (Keep chips warm in a 250 degree oven while cooking fish.)
Dip 2 cod fillets in reserved batter and slowly ease them into oil. Fry until golden brown and cooked through, about 7 minutes; drain well on paper towels. Repeat with last 2 fillets. Serve hot with extra salt and vinegar on the side.