Try this one, I've made it a few times and it always works:
500g medium raw prawns
6 spring onions
1 tbsp oil
3 cloves garlic, finely chopped
2 tbsps caramel sauce*
1 tbsp fish sauce
1 tbsp lime juice
1 tbsp soft brown sugar
1/2 tsp salt
1/4 red pepper, cut into fine strips, to garnish
1. Remove the prawn heads and, using a fine needle, devein prawns, leaving the tails, shells and legs intact. Rinse the prawns under running water and pat dry with paper towels.
2. Finely chop half the spring onions. Cut the rest into 4 cm long pieces and then finely shred the pieces into thin strips.
3. Heat the oil in a heavy-based frying pan; add the garlic, chopped spring onion and prawns, and cook over medium heat for about 3 minutes, tossing the prawns until they turn pink. Drizzle the caramel sauce and fish sauce over the top and cook for 1 minute. Add the lime juice, sugar, salt and remaining spring onion. Toss well and serve immediately, garnished with the red pepper.
* To make the caramel sauce, combine 4 tablespoons of sugar with 3 tablespoons of water in a small pan. Stir over low heat, without boiling, until the sugar has dissolved. Bring the syrup to the boil, reduce the heat and simmer gently for about 5 minutes, until the syrup turns dark golden. Take care not to burn it. Remove the pan from the heat and add 4 tablespoons of water – it will spit and sizzle, and the caramel will form hard lumps. Return the pan to the heat and cook, stirring, until the lumps become liquid again. The sauce can be stored in the refrigerator for up to 1 week.