Spread mustard on both sides of chicken and sprinkle with pepper. Cover and refrigerate 2 hours. Melt butter, stir in lemon juice, garlic powder and tarragon. Cook over low heat for 5 minutes, stirring occasionally. Place chicken on grill over medium coals or heat. Baste with sauce and grill 50 to 55 minutes or until done, turning and basting every 10 minutes.