Fill a large pot with enough oil to reach a depth of 2 inches and heat until it registers 375 degrees on a deep-fry thermometer. In a large bowl, whisk together the flour, granulated sugar and salt; form a well in the center of the mixture. Add 1/4 cup cider and the whole egg. Gradually stir the flour into the liquid to form a smooth paste. Stir in the remaining 1/4 cup cider; let stand until thickened, 5 minutes. In a bowl, beat the egg whites until stiff. Fold into the batter.
Working in batches, dip the apple rings in the batter and fry until golden, about 3 minutes, allowing the oil temperature to return to 375 degrees between batches. Using a slotted spoon, transfer to paper towels to drain. Dust with the confectioners' sugar and serve warm.