01 - If using fresh pineapple, peel and core the fruit, then slice crosswise into 1/2-inch thick rings. Pat each ring thoroughly dry with paper towels to remove excess moisture, which helps the batter adhere properly during frying.
02 - In a medium mixing bowl, whisk together the all-purpose flour, cornstarch, granulated sugar, baking powder, and salt until well combined. Pour in the cold sparkling water and add the egg if using. Stir gently until just combined—the batter should remain slightly lumpy. Avoid overmixing, which can make the coating tough.
03 - Pour vegetable oil into a deep skillet or heavy saucepan to a depth of about 2 inches. Heat over medium-high heat until the temperature reaches 350°F. Use a kitchen thermometer to monitor the oil temperature for consistent results.
04 - Working with one ring at a time, dip each pineapple slice into the batter, turning to coat completely. Lift and allow excess batter to drip off for a few seconds before transferring to the hot oil.
05 - Carefully lower the battered pineapple rings into the hot oil, working in batches to avoid overcrowding the pan. Fry for 1 to 2 minutes on the first side, then flip and cook for another 1 to 2 minutes until the coating is golden brown and crispy. Remove with a slotted spoon and transfer to paper towels to drain excess oil.
06 - Dust the warm fried pineapple rings generously with powdered sugar and sprinkle with ground cinnamon if desired. Serve immediately while still crispy, optionally accompanied by vanilla ice cream or warm caramel sauce for dipping.