Combine olive oil and 450ml water in a saucepan and bring to the boil over medium-high heat. Gradually add flour and a pinch of salt and beat with a wooden spoon to combine. Transfer to an electric mixer fitted with the paddle attachment and beat until a very smooth dough forms (3-4 minutes). Transfer dough to a piping bag fitted with a 1cm star nozzle, pushing dough down to remove any air bubbles. Refrigerate to rest and firm up (20 minutes).
Pipe long lengths of dough (about 20cm) into large rings onto a sheet of baking paper.
Heat oil in a deep-fryer or a large, deep-sided saucepan to 180°C. Fry dough rings in batches, turning occasionally, until golden (3-5 minutes; be careful, hot oil will spit). Remove with a slotted spoon, drain on paper towels and dredge with caster sugar, shaking off excess. Serve warm with chocolate sauce.
Bring sugar and 150ml water to a simmer in a saucepan over medium heat, stirring to dissolve sugar. Add chocolate and cocoa and stir until chocolate has melted and sauce is smooth. Strain through a fine sieve and refrigerate to chill (1 hour).
