900 g whole prawns (yields 170 g approx. prawn tails)
Bring the water to the boil and add the salt. Remove the heads of the prawns and discard or use for making fish stock. With the underside of the prawn uppermost, tug the little fan-shaped tail at either side and carefully draw out the trail. (The trail is the intestine, so it is very important to remove it before cooking regardless of whether the prawns are to be shelled or not.)
Put the prawns into the boiling salted water and as soon as the water returns to the boil, test a prawn to see if it is cooked. It should be firm and white, not opaque or mushy. If cooked, remove prawns immediately. Very large ones may take ½ to 1 minute more. Allow to cool in a single layer and then remove the shells.