Combine flour, sugar and a pinch of salt in an electric mixer fitted with a paddle attachment.
Bring oil and 450ml water to the boil in a saucepan over high heat, then, with the motor running on low speed, gradually add to flour until all incorporated. Increase speed to high and beat until a very smooth dough forms (2-3 minutes). Transfer dough to a piping bag fitted with a 1.5cm star nozzle and push down to remove any air bubbles in the bag. Refrigerate to rest for 30 minutes.
Heat oil in a deep-fryer or deep saucepan to 180C. Working in batches of 3-4, pipe lengths of mixture (about 15cm-20cm) into hot oil, cutting off dough with scissors (be careful, hot oil will spit). Deep-fry, turning occasionally, until golden (3-5 minutes), then remove and drain on paper towels. Toss in caster sugar, shaking off excess, and serve hot with dulce de leche for dipping.
