Combine the soy sauce, garlic, ginger, green onions, cilantro stems, red pepper flakes, sesame oil, and brown sugar in a blender or food processor. Blend until smooth.
Place the chicken pieces in a shallow dish and pour the marinade over them. Toss to coat the chicken evenly. Cover and refrigerate for at least 2 hours or overnight.
Preheat the grill to medium-high heat. Thread the marinated chicken and mushrooms onto skewers, alternating between the two.
Grill the skewers for 10-12 minutes, turning occasionally, until the chicken is cooked through and the mushrooms are tender.
While the skewers are grilling, combine the remaining marinade and hoisin sauce in a small saucepan. Heat over medium heat until warmed through.
Brush the glaze onto the skewers during the last few minutes of grilling.
Serve the skewers hot, garnished with chopped cilantro leaves if desired.
