Mix all ingredients for cooking sauce and set aside.
In bowl, combine cornstarch, sherry, water, soy sauce, 1 Tablespoon oil, salt and pepper. Pour over chicken, stir to coat and set aside for 15 minutes.
Heat wok or large skillet over high heat.
Add 2 Tablespoons oil to pan. Add chicken with marinade and stir fry until done, about 3-4 minutes; set chicken aside.
Add 2 Tablespoons oil to wok and add water chesnuts, mushrooms, green onions, garlic and ginger. Stir fry 2 minutes.
Add chicken and cooking sauce with vegetables. Cook until thickened. Serve with lettuce leaves to wrap.