Step 1Heat the oven to 400 degrees and place a rack on the top third and bottom third of the oven.
Step 2In a spice grinder, grind the peppercorns, allspice, bay leaves, red pepper flakes and paprika into a fine powder.
Step 3Place the spot prawns in a large bowl, pour the spice mixture over and toss to combine. Place the salt in a separate large bowl and stir in 1 2/3 cups water so the texture is that of gritty snow.
Step 4Line two jellyroll pans with foil and spread some of the salt in a layer about one-fourth-inch deep. Divide the prawns evenly between the pans, arranging them in a single layer. Mound the rest of the salt on top, covering them completely in a smooth, even layer.
Step 5Roast for 18 minutes until the prawns are fragrant and the meat is firm. Remove the pans from the oven and cool briefly.
Step 6With a sturdy metal spoon or chef's knife, chip a crack around the base of the salt crust and carefully lift off the top. Use a dry pastry brush to wipe away any salt on the surface of the prawns. Serve immediately with fingerbowls for rinsing.