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 and pour it in a steady stream on to the egg yolk, whisking continuously until the sauce thickens to a light coating consistency as with a Hollandaise. Keep warm in a flask or place in a pottery or plastic bowl (not stainless steel) in a saucepan of hot but not boiling water.
Just before serving, toss the prawns in foaming butter until heated through. Heap them on to a hot serving dish. Coat with Bretonne sauce. Garnish with flat parsley or fresh fennel and serve immediately.
© 2001 Darina Allen. All rights reserved.