Heat oven to 200 degrees. Set wire rack inside a rimmed baking sheet. Pat fish dry with paper towels and season with salt and pepper. Whisk flour, cornstarch, baking powder and 1 teaspoon salt together in a large bowl. Add beer and whisk until smooth Transfer fish to batter and toss until evenly coated.
Add oil to large Dutch oven until it measures about 3/4 inch deep and heat to 350 degrees.
Working with 5 to 6 pieces at a time, remove fish from batter, allowing excess to drip back into bowl, and add to hot oil. Fry fish, stirring gently to prevent pieces from sticking together, until golden brown and crispy, about 2 minutes a side. Transfer fish to prepared wire rack and place in oven to keep warm