1. Mix 2 tablespoons each sliced scallion and chopped chestnuts with the duck sauce in small bowl.
2. Heat oil in medium-size nonstick skillet over medium-high heat. Add mushrooms, garlic and remaining scallion. Stir-fry until mushrooms are tender and scallion is soft, about 3 to 4 minutes. Let cool slightly.
3. Gently combine ground chicken, mushroom mixture, remaining chestnuts, chopped cilantro and soy sauce in a bowl. Shape into 4 patties about 3/4 inch thick. Season tops with half the salt and pepper.
4. Add patties to same skillet, seasoned side down. Sprinkle with remaining salt and pepper and cook over medium heat for 9 minutes. Gently turn patties. Cook 8 to 9 minutes longer, or until they register 165 degrees F on an instant-read thermometer.
5. Fill each bun with a burger. Top with sauce and cilantro sprig. Pass any extra sauce.