In a large bowl, whisk together the flour, sugar, baking powder and salt. Add the water and egg and whisk until the batter is smooth.
Peel plantains and cut on a slight diagonal into 1/2-inch pieces. Heat a half inch of oil in a heavy skillet or pot until hot, but not smoking. Dip the plantains in the batter to coat them and fry in batches (don't crowd them) until bottoms are golden, about 45 seconds, then turn over and fry until other side of each is golden, 30 to 45 seconds more. Transfer to paper towels to drain. Stir together the sugar and cinnamon in a shallow bowl.
While still warm, toss each batch in sugar mixture until coated.