In a large bowl mix together the cornflour, salt and coriander. Add the king prawns and toss until coated.
Heat the vegetable oil in a deep, heavy-bottomed saucepan. It is ready when a breadcrumb which is dropped in sizzles gently in it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Deep fry the prawns for 30 seconds, then drain on kitchen towels.
Serve the prawns in a warm bowl.