Put the soy sauce, sherry, sesame oil, garlic, chilli flakes and sugar in a bowl. Whisk together with a fork, add the tofu cubes then toss to coat in the mixture. Cover and set aside. (Leave to marinate if convenient.)
Bring a large saucepan of water to the boil, add the noodles and boil for 5 minutes until softened. Tip into a colander and drain well.
While the noodles are cooking, heat a wok or large, deep frying pan until hot then add the oil. Drain the tofu, reserving the marinade then stir-fry in the hot oil for about 30 seconds. Add the pak choy and water chestnuts and cook for a further minute, stirring all the time.
Tip in the cooked noodles and reserved marinade. Cook for a further 1–2 minutes or until heated through and the pak choy has wilted. Serve sprinkled with sesame seeds.