Rinse the prawns and pat dry. Slice the garlic (a mandolin is best) into thin slices and set aside.
Heat a large cast iron skillet over medium-high heat for 3 minutes (to let it get really hot).
Sprinkle half of the salt onto pan and let heat for a minute more (you will want to turn on your exhaust fan).
Sprinkle half of the garlic slices over salt and place half of the shrimp immediately on the garlic.
Let the prawns cook on one side, without moving, until pink is visible, about 3 minutes.
Turn prawns over and cook 3 minutes more. Remove prawns and serve warm. Repeat with remaining prawns.
To eat, remove the head and peel the prawns. As you peel, the toasted garlic and salt will stick to your fingers and season the prawn as you eat them.