Preparation

• Put the chicken in a medium bowl and toss with 2 tsp. of the soy sauce, the sake, cornstarch, and 1/4 tsp. salt. Heat a wok or a 12-inch skillet over high heat. Add 1 Tbs. of the oil and the chicken to the pan in a single layer. Leave the chicken undisturbed for about a minute before stirring, and then cook, stirring occasionally, until browned and partially cooked, about 4 minutes. Transfer the chicken to a bowl.
• Return the pan to high heat. Add the remaining 1 Tbs. oil, the chile-bean sauce, and ginger to the pan, and stir to combine. Add the mushrooms, and cook, stirring frequently, until tender, about 2 minutes.
• Return the chicken and any accumulated juices to the pan. Stir in the broth and the remaining 1 tsp. soy sauce. Cook, stirring frequently, until the chicken is cooked through, about 2 minutes. Remove the pan from the heat and stir in the watercress, vinegar, and more chile-bean sauce to taste. Toss until the watercress wilts. Serve hot.