Combine sesame seeds, ginger, honey, and soy sauce in a small bowl; stir well and set aside. Place chicken between 2 sheets of heavy duty plastic wrap, and flatten to 1/4 inch thickness, using mallet or rolling pin.
Coat grill rack with cooking spray; place on grill over medium hot coals. Place chicken on rack and cook 4 minutes on each side, basting frequently with soy sauce mixture. Transfer chicken to a serving platter; garnish with green onion, if desired.