01 - If using fresh pineapple, peel, core, and slice crosswise into 1/2-inch thick rings. Pat slices thoroughly dry with paper towels to remove excess moisture, which helps the batter adhere properly.
02 - In a medium mixing bowl, whisk together flour, cornstarch, sugar, baking powder, and salt until well combined. Pour in cold sparkling water and add egg if using. Stir gently until just combined—small lumps are fine. Do not overmix as this toughens the batter.
03 - Pour vegetable oil into a deep skillet or heavy saucepan to a depth of 2 inches. Heat over medium-high heat until the temperature reaches 350°F. Use a kitchen thermometer for accuracy, or test by dropping a small amount of batter—it should sizzle immediately and rise to the surface.
04 - Working with one ring at a time, dip pineapple into the batter, turning to coat completely. Lift and allow excess batter to drip off for a few seconds. A thin, even coating yields the crispiest result.
05 - Carefully lower battered rings into the hot oil, working in batches to avoid overcrowding. Fry for 1 to 2 minutes per side until deep golden brown and crispy. Remove with a slotted spoon and transfer to a paper towel-lined plate to drain excess oil.
06 - While still warm, generously dust fried pineapple rings with powdered sugar. Add a light sprinkle of ground cinnamon if desired. Serve immediately for best texture, accompanied by vanilla ice cream or warm caramel sauce if preferred.