Slice the peeled and cored pineapple into 1/2-inch thick rings.
In a mixing bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually add sparkling water, whisking until a smooth batter forms.
Heat vegetable oil in a deep pan to 350°F (175°C).
Dip each pineapple ring into the batter, letting excess drip off.
Carefully place into the hot oil and fry until golden brown on both sides, about 2 minutes per side.
Remove and drain on paper towels.
Dust with powdered sugar and sprinkle with cinnamon if desired.
Serve warm and enjoy!