Classic Fish and Chips

Name: Classic Fish and Chips
Ingredients: 3 large russet potatoes, scrubbed clean; Canola oil, for frying; Kosher salt; 1/2 cup (65 grams) all-purpose flour; 1/2 cup (65 grams) cornstarch; Four 7-ounce (200-gram) haddock or halibut fillets; 1 teaspoon baking powder; 2/3 cup (167 milliliters) cold light beer or sparkling water; 1/2 cup (125 milliliters) mayonnaise; 1/2 lemon, zested and juiced; 3 tablespoons chopped dill pickles; 2 tablespoons capers, chopped; 2 tablespoons finely chopped fresh parsley; 2 tablespoons finely chopped fresh dill; Freshly ground black pepper; Malt vinegar, for serving