Bring the water to the boil and add the salt. De-vein the prawns. Toss them into the boiling water and as soon as the water returns to the boil, test a prawn and then remove immediately. Very large prawns may take ½–1 minute more. Allow to cool and then remove the shells.
Next make the Bretonne sauce. Whisk the egg yolk with the mustard and herbs in a bowl. Bring the butter to the boil