Combine the cornflour, fish sauce and soy sauce to make a paste. Stir in the sugar and stock and set aside.
Cook the noodles in a large pan of boiling water, according to the packet instructions. Refresh in cold running water, then drain thoroughly.
Pour enough oil into a wok to deep-fry the noodles, and heat it to 190°C (375°F), or until a cube of bread dropped into the oil turns golden in 20 seconds. Add half the noodles and deep-fry they are until crisp and golden, then remove using a slotted spoon and drain on kitchen paper. Stir-fry the remaining noodles in the same way. Place on a warm serving dish.
Pour off all but 2 tablespoons of the oil and place the wok over a high heat. Add the garlic and chicken and stir-fry for 2 minutes, then tip in the remaining ingredients, except for the coriander. Stir-fry for a further 2 minutes, then pour in the prepared sauce and bring to the boil. As soon as the sauce starts to thicken, spoon the stir-fry over the crispy noodles and garnish with coriander sprigs.