Cook the noodles following pack instructions, then rinse with cold water and drain. Toss with 1 tsp of the sesame oil.
Heat 2 tbsp of the groundnut oil in a non-stick wok. Stir-fry most of the spring onions and all the beansprouts for a couple of mins until tender. Add the noodles and warm through. Stir through the remaining sesame oil and tip out of the wok onto a serving dish.
Carefully wipe out the wok and add the remaining groundnut oil. Toss in the garlic and chilli, and cook for 10 secs. Pop in the prawns and stir-fry for a couple of mins until they have just turned pink. Stir in the sugar and soy, then bubble until the sugar has melted and prawns are cooked through. Spoon on top of the noodles and sprinkle with the remaining spring onions. Add an extra drizzle of sesame oil, if you like.