1. Preheat oven to 425 degrees F. Coat 12 2 1/2-inch muffin cups with cooking spray; set aside. In a medium bowl stir together 1/4 cup of the flour and the cinnamon. Add banana pieces and toss to coat (you may not use all of the flour mixture). Transfer banana pieces to a plate or tray and freeze for 10 minutes.
2. Add the remaining 1/2 cup flour to another medium bowl. Slowly whisk in the sparkling water until fully incorporated. Whisk in the egg whites, vanilla, and granulated sugar until you have a smooth batter.
3. Add the banana pieces, one at a time to the batter, turning to coat. Using a spoon, spoon the banana pieces and some of the batter into the prepared muffin cups. Spoon any remaining batter evenly over the banana pieces.
4. Bake for 7 minutes or until batter is set. Invert the muffin cups over a tray to remove the fritters. Sprinkle with powdered sugar and serve warm.