Beat together egg and milk. Stir in cornstarch, flour and baking powder until well blended. Pat fish pieces dry with paper towelling. Dip fish into batter to coat evenly and completely. Fry in 375 F cooking oil in deep fryer or frying pan for about 8 minutes, or until golden, turning once. Fish will rise to surface when done, drain well on towelling. Sprinkle lightly with salt and pepper. Serve immediately. (This is the best recipe I have used for deep-fried fish; my kids loved it. I also dipped and deep-fried onion rings with any leftover batter to serve with the fish; good!)