Barbecued King Prawns with Lemongrass Butter

Name: Barbecued King Prawns with Lemongrass Butter
Prep time: 20 min
Cook time: 20 min
Total time: 40 min
Yield: 2-3
Ingredients: 10 x large king prawns, shell on; 1 tbsp vegetable oil; Nuoc cham dressing; 3 tbsp fish sauce; 2 tbsp white vinegar; 3 tbsp sugar; 2 tbsp lime juice; 1 long red chilli, finely chopped; 2 garlic cloves, finely chopped; Lemongrass butter; 120g (4 oz) unsalted butter; 1 lemongrass stalk, white part only, finely chopped; zest of 1 lime; 2 tbsp finely chopped coriander leaves; 2 tsp sea salt flakes