Heat a wok or large saucepan over medium heat with the vegetable oil. Break up the chicken into the pan. Add the sake, mirin, chicken stock, soy sauce, sugar, ginger, and salt. Use a pair of chopsticks or a wooden spoon to continuously stir the chicken as it cooks, breaking it up into fine pieces. Continue stirring until only a little liquid remains to keep chicken moist. Remove chicken from pan and cover to keep warm.