For the Marinade
In a shallow bowl, combine sherry, soy sauce, brown sugar, garlic, ginger, and chiles; stir well. Stir in chicken, cover and refrigerate at least 1 hour. Lift from bowl with a slotted spoon; measure marinade and add enough stock to make 1 1/4 cups. Set aside.
In a wok or large frying pan, heat sesame and peanut oil over high heat. Add chicken and stir fry until browned (4 minutes). Add green onions, mushrooms, broccoli, and black beans; stir fry until broccoli is tender-crisp (5 minutes).
In a small cup, mix cornstarch and water together. Add cornstarch mixture to stock-marinade mixture. Pour into pan and cook, stirring constantly, until sauce is thickened.