Preheat grill to medium heat. Sprinkle chicken with salt and 1/2 tsp pepper.
Grill chicken for 5-6 minutes per side, or until cooked through and juices run clear.
While chicken is cooking, combine mustard, honey, and remaining 1/4 tsp pepper. Spread butter on cut sides of buns.
Remove chicken from grill; keep warm. Place buns buttered sides down on grill. Grill 2-3 minutes or until toasted.
Assemble sandwiches: bottom bun, chicken, tomato, lettuce, honey mustard, and top bun.