Put a thin layer of rock salt into a frying pan or on a griddle, as if you are making a salty bed, then lay the prawns on top.
Put on a medium heat and cook for 1 minute for small prawns or 2–3 minutes for larger prawns. Look at the side of the prawn as it cooks: when the line showing that the colour has changed is halfway up the prawn, it’s time to turn them.
Shake them off in the pan to remove any excess salt, and pile on to a plate.