2) In a medium bowl, whisk together the lime juice and sugar until the sugar is completely dissolved, then add the fish sauce. Add the garlic, chilli, Thai basil and 1 tbsp of the oil. Stir well and pour over the reserved prawns in the plastic bag. Seal the bag and leave to marinate for at least an hour.
3) Heat a grill pan over high heat and add the remaining oil. Grill the prawns, turning once to prevent burning, until evenly pink and golden on both sides, 2 to 3 minutes. Serve the prawns on a large plate over sour mango salad.
For the sour mango salad:
1) Mix the mangos, carrots, and chillis in a large bowl.
2) For the dressing, in a medium bowl, whisk together the lime, fish sauce, and sugar until the sugar is completely dissolved.
3) Add the dressing to the bowl and toss together. Leave to stand for 20 minutes. Drain and serve at room temperature or lightly chilled.
Cook's Note: Not all ingredients are created equal, so adjust the seasoning as you go. Taste the dressing prior to adding to the ingredients to make sure the sour, salty and sweet notes are balanced. Adjust the seasoning according to the acidity and sugar.