1) Preheat a griddle pan over medium-high heat.
2) Loosen shells of prawns and butterfly them, cutting down the devein line of the back of each prawn. Toss prawns with 1 teaspoon coarse salt.
3) Combine melted butter with lemon juice and zest. Using a pastry brush, cover prawns with lemon butter.
4) Griddle prawns for 3 to 4 minutes on each side, until pink and firm.
5) Arrange 5 griddled prawns down the centre of one leaf of cos lettuce.
Serve prawns with seafood forks alongside dipping sauce.
Horseradish dipping sauce:
1) In a bowl, combine breadcrumbs, horseradish, cream and salt. Let the cream soak into the breadcrumbs for about 2 minutes.
2) Loosen the breadcrumbs with a fork. Stir in cayenne sauce or cayenne and combine with sour cream. Spoon equal amounts of sauce into ramekins on individual plates or a dip bowl in the centre of one large platter.