Mix the wasabi, soy sauce and garlic in a small bowl. Bring a large pan of water to the boil and cook the noodles following pack instructions.
Meanwhile, heat the butter in a frying pan. Once foaming, stir in the prawns and cook for a few mins until pink. Stir in the wasabi mixture with a couple of spoonfuls of the noodle cooking water and heat through. Add the noodles to the pan with the spring onions, toss together and divide between two plates.