In a small bowl, whisk together the tamari, maple syrup, five-spice, pepper and cayenne (if using). Using a sharp knife, score the duck skin in a 3/4-inch diamond pattern, taking care not to cut into the flesh. Rub the sauce evenly over all surfaces of the duck. Cover and refrigerate for at least 4 hours or up to one day.
Preheat a gas grill on medium. Turn off the centre burner. Grease the grill. Place the duck on the centre of the grill, skin-side down, and cook, lid down, for 8 minutes. Turn the duck over and grill for 7 minutes, lid down. Transfer the duck, skin-side down to the lit side of the grill and cook over direct heat for 5 minutes, keeping an eye on possible flareups. Allow the duck to rest for about ten minutes before slicing.
Makes 4 servings