Combine the sugar and cinnamon in a bowl and stir.
Heat the oil to 350°F in a deep fryer.
Place the duros into the fryer, about 15 at a time. Nothing will happen for the first few seconds. Then they will suddenly puff up. Continue cooking for 5 seconds after they have puffed up.
Remove the duros from the oil and transfer them to a wire rack to drain.
Sprinkle the duros liberally with cinnamon and sugar.
Video
Recipe Tips
If you don't have a deep fryer you can fry the duros in a heavy-bottomed pot or Dutch oven. Use a candy or deep fry thermometer to monitor the oil temperature.