Stack two 18x12-inch pieces of heavy-duty foil; fold edges over, and bring up sides to make a basket. Spray foil with cooking spray. Place foil on grill over medium heat; place chicken on foil. Cover grill; cook 4 minutes. Turn; cook 4 minutes, then turn again, cooking 3 to 6 more minutes or until juice of chicken is clear when center of thickest part is cut (at least 165°F). Remove toothpicks. Serve chicken topped with ranch dressing and dill leaves.