1 In a saucepan bring the coconut water to a gentle simmer and add the lemongrass. Place a bamboo steamer over the saucepan and add the prawns and steam for 5-8 minutes, until cooked through. Once cooked, leave to cool and then peel and halve lengthways. Add the noodles to the boiling coconut water and stand for 3 minutes until tender, drain and cool under cold water.
2 Lightly moisten the rice papers, spreading some water over the surface by hand and then store under a damp towel.
3 Place a rice paper on a damp towel and put a lettuce leaf on top, centred and covering about a third of the bottom of the rice paper. On top of the lettuce, add one-eighth of the noodles, coriander and mint leaves.
4 Roll the rice paper halfway into a cylinder, pressing down while you are folding in order to make it tight. Fold 3.5cm of the 2 ends of the
paper over the filling. Lay 2-3 prawn halves along the crease, with the cut side downwards.
5 Place 2 chive sprigs on top of the prawn at one end. Leave about 1-2cm of the chives extending from the roll. Continue rolling until you complete the cylinder, and seal by putting a bit of water on your finger and wetting the leading edge of the paper.
6 Repeat with the remaining ingredients to make 7 more rolls in the same way.
7 You can cut the rolls in half and stand the halves end up, with the chives sticking out of it for a traditional presentation.
8 To make the sauce, mix the vinegar, sugar and 60ml water. Bring to the boil and set aside to cool. When cool, combine with the chilli, garlic and lime juice. Mix well and stir in the fish sauce.