For the allioli, put the egg yolk, garlic and salt in a food processor and blitz on medium speed for 3-4mins, until creamy. With the motor still going, add the sunflower oil and olive oil in a slow, steady stream, until the sauce is emulsified. Towards the end, once the mix has started to come together, you should be able to add it a little faster. Stir in the lemon juice, then set aside (or cover and put in the fridge, where it will keep for up to 5 days).
Just before you’re ready to cook the prawns, stir the cornflour and egg into the prawn mixture, and season. Heat a good amount of oil in a deep-fat fryer (or high-sided frying pan) to 170°C. Working in batches of 4, to not overcrowd the fryer, drop tablespoons of the prawn batter into the hot oil and fry for two mins, until golden brown. Drain the cooked fritters on kitchen paper and keep warm in a low oven while you fry the remaining batter.
Serve the fritters hot with a garnish of chopped coriander and a bowl of allioli for dipping.