Place chicken between 2 pieces of plastic wrap and pound with a meat mallet into ½"-thick pieces. Transfer chicken to a large bowl; season all over with 1 teaspoon salt and ¼ teaspoon black pepper. Pour ½ 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, ¾ teaspoon salt, and ¼ 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 ¼ cup BBQ sauce and toss again to combine. Serve remaining dressing alongside.
Recipe by 5* trending recipes with videos at https://food.theffeed.com/2023/04/01/bbq-chicken-salad/