Use kitchen scissors to make a lengthwise slit along the back of each prawn to expose the flesh. With a paring knife, cut almost all the way through the flesh down the centre of each prawn. Pull out or rinse the dark intestinal vein. Pat the prawns dry. Open the flesh of each prawn and press on it until it sits flat. Set the prawns, flesh side up, in a wide baking dish or large ovenproof skillet.
Place the butter, garlic, chili flakes, wine, lemon juice and paprika in a pot. Set over medium heat, cook just until the butter is melted, and then simmer 1 minute. Spoon some of the mixture over each prawn.
Set the oven rack 6 inches below the broiler. Preheat the broiler to high. Broil the prawns 3 to 4 minutes, or until just cooked through. Sprinkle with parsley, garnish with lemon slices and serve.