Set a colander in a large bowl of ice water. Slice the potatoes lengthwise 1/4 inch thick, then stack the slices and cut into 1/4-inch-thick sticks, adding them to the ice water as you work. Refrigerate 30 minutes or up to 1 hour. Lift the colander out of the ice water, rinse the potatoes well and pat very dry.
Preheat the oven to 350 degrees F. Heat 4 inches vegetable oil in a large Dutch oven until a deep-fry thermometer registers 280 degrees F. Add half of the potatoes; fry until tender but still colorless, about 5 minutes. Remove with a slotted spoon to a paper towel-lined plate. Repeat with the remaining potatoes. Increase the oil temperature to 365 degrees F.
Whisk 3/4 cup all-purpose flour, the rice flour, baking powder, 1 teaspoon salt and the cayenne in a large bowl. Whisk in the beer until mostly smooth. Let sit 5 minutes to thicken.
Set a wire rack on a rimmed baking sheet. Season the fish with salt. Working in two batches, dredge the fish in the remaining 1/2 cup all-purpose flour, then dip in the beer batter, letting the excess drip off. Fry until crisp and golden brown, about 4 minutes. Transfer to the rack; sprinkle with salt. Keep warm in the oven.
Increase the oil temperature to 380 degrees F. Working in two batches, fry the potatoes again until crisp and golden brown, about 2 minutes. Drain on a paper towel-lined plate and season with salt. Serve the fish and chips with malt vinegar and lemon wedges.
Photograph by Levi Brown
A combination of rice flour and all-purpose flour in the batter makes the coating on the fish light and crisp.