Place the cakes of udon noodles into a pot of boiling water. Use tongs to gently separate the noodles and as soon as they have come apart, transfer the noodles to a large bowl. Toss with sesame oil.
Heat the vegetable oil in a wok or frying pan over high heat. Add the cabbage and stir-fry until just starting to char. Then add the ½ tsp of sugar and the ½ tbsp of soy sauce. Stir-fry for another minute or until the cabbage is nicely coloured. Add the garlic and stir-fry for 20 seconds. Then add the prawns and chilli powder and stir-fry until the prawns are just cooked.
Add the noodles, remaining soy sauce, remaining sugar and oyster sauce. Stir-fry for another half a minute until the sauce has thickened. Toss through the spring onion and serve.
