You'll need two medium bowls. In the first, whisk together the flour, cayenne, salt, pepper and baking powder. In the second whisk together the milk and egg.
Heat 2"-3" of oil to 375F in a deep fryer or Dutch oven.
Meanwhile, place the separated onion rings into the flour mixture. Using your hands, gently coat the rings in the flour.
Transfer the rings to the milk and dunk and turn to coat well.
Drain well and transfer the rings back to the flour mixture. Using your hands again, gently coat the rings in the flour.
Working in batches as needed, deep-fry the rings until golden brown. This will take 2-3 minutes.
Transfer to a wire rack or paper towel to drain. Season immediately with more salt.
Serve hot.
Notes
Season the rings with more salt right out of the fryer.