Pour the coconut water into a pot. Add lemongrass (cut into pieces and lightly smashed), cleaned cilantro stems, salt, and sugar. Clean the prawns. When the coconut water comes to a boil, add the prawns.

Turn the heat to high and quickly stir so the prawns absorb the coconut water and seasonings.

Remove the prawns to a plate. Serve with a dipping sauce of salt, pepper, and lime, or green chili salt, or sweet and sour fish sauce.