Peel only the body of the prawns, leaving the heads and tails intact. Devein them.
In a small bowl, combine the sugar and fish sauce and stir until the sugar has dissolved. Divide the fish sauce mixture into 2 halves, and use 1/2 of the mixture to marinate the prawns for 10 minutes.
Place a large frying pan over medium-high heat, add 1 tablespoon of the oil, then sear the prawns for 1 minute on each side. Remove the prawns and set aside.
Wipe the frying pan clean, return the pan to medium-high heat and add the remaining 1 tablespoon oil and the garlic and shallots; fry for 1 minute, or until fragrant. Add the coconut milk and reserved fish sauce mixture, and stir.
Return the prawns to the pan, tossing them to coat in the sauce. When the liquid begins to boil, take off the heat. Garnish with black pepper, coriander and chilli. Serve with steamed jasmine rice.