Pat pineapple rings completely dry on both sides using paper towels.
Whisk flour, cornstarch, sugar, baking powder, salt, and cinnamon together in a bowl.
Add egg, vanilla, and cold water. Whisk until just combined with a few small lumps remaining.
Heat oil in a heavy pot to 370°F.
Dip each pineapple ring into the batter, let excess drip off, then lower gently into the hot oil.
Fry 2 to 3 rings at a time for 2 to 3 minutes, turning once, until deep golden. Drain on a wire rack.
Dust with powdered sugar while hot and serve immediately.