Whip the egg whites to soft peaks in a large bowl. Fold in the moonshine or vodka. Sift the cornstarch, salt, black pepper, and cayenne over the egg whites and gently fold to combine.
Line a baking sheet with paper towels. Place a wire rack near the baking sheet. Spread the panko, if using, evenly on a second baking sheet.
Working in batches, add the onion rings to the batter. Using a fork or a slotted spoon, remove the rings from the batter, one at a time, and allow any excess to drain off. If desired, toss the onion rings in the panko and flip them over to coat. One by one, carefully drop the coated onions into the hot oil without crowding them and cook them in small batches until golden brown, about 3 minutes. Remove the onion rings with a slotted spoon and transfer to the lined baking sheet to drain briefly. Sprinkle with salt and then transfer to the wire rack. Repeat with the remaining onion rings.
Serve 'em steaming hot from the oil or still pretty hot.