2 boneless, skinless chicken breasts (about 1 lb. total)
Freshly ground black pepper
3/4 c. store-bought BBQ sauce, divided
1/3 c. buttermilk
1/4 c. mayonnaise
1/4 c. sour cream
1 clove garlic, finely chopped
2 tbsp. chopped fresh parsley
1 tbsp. chopped fresh dill
1 tbsp. sliced fresh chives
1/4 tsp. onion powder
Pinch of cayenne pepper
2 heads romaine lettuce, thinly sliced (about 8 c.)
1 (15-oz.) can black beans, rinsed, drained
1 vine-ripened tomato, finely chopped (about 1 c.)
1 c. shredded Mexican blend cheese
1 c. frozen corn, thawed, drained
1 c. tortilla strips or crushed tortilla chips
1/4 c. finely chopped red onion
Place chicken between 2 pieces of plastic wrap and pound with a meat mallet into 1/2″-thick pieces. Transfer chicken to a large bowl; season all over with 1 teaspoon salt and 1/4 teaspoon black pepper. Pour 1/2 cup BBQ sauce over chicken. Turn to coat and let marinate, tossing a few times, at least 15 minutes at room temperature, or cover bowl and refrigerate up to 3 hours.
Meanwhile, prepare a grill for medium-high heat; heat 5 minutes (or preheat a grill pan over medium-high heat). In a medium bowl, whisk buttermilk, mayonnaise, sour cream, garlic, parsley, dill, chives, onion powder, cayenne, 3/4 teaspoon salt, and 1/4 teaspoon black pepper; set dressing aside.
Grill chicken, basting with marinade, until cooked through, about 5 minutes per side. Transfer to a cutting board and let cool slightly. Cut chicken into bite-sized pieces.
In a large bowl, toss chicken, lettuce, beans, tomatoes, cheese, corn, tortilla strips, and onion. Drizzle with some of the reserved dressing and remaining 1/4 cup BBQ sauce and toss again to combine. Serve remaining dressing alongside.