Preheat oven to 425 degrees Fahrenheit. Place chicken in a baking pan.
Combine 3/4 cup soy sauce, brown sugar, 1/4 cup ginger, and garlic with four quarts water. Mix well and pour over chicken. Roast for 30 minutes.
In a saucepan, combine remaining 1/4 cup soy sauce, two tablespoons ginger, cilantro, chicken broth, rice vinegar, hoisin sauce, mirin, sesame oil, scallions, and mushrooms. Bring to a boil and let cook for 20 minutes, until reduced and thickened.
Remove chicken from oven. Pour sauce over chicken. Return chicken to oven for five minutes. Let rest for five minutes before serving. Serve hot, garnished with additional fresh cilantro.