In a medium bowl, combine the shredded or cubed rotisserie chicken with 1/4 cup BBQ sauce until the chicken is evenly coated; set aside.
In a large bowl, add drained corn, drained and rinsed black beans, chopped tomatoes, cubed avocados, chopped romaine hearts, and chopped cilantro; gently toss to combine.
In a small bowl, whisk together buttermilk, 1/4 cup BBQ sauce, ranch dressing, and dill until smooth; season with salt and pepper to taste.
Add the BBQ-coated chicken to the salad bowl, pour the BBQ-ranch dressing over the top, and toss until everything is evenly coated.
Serve immediately, or cover and refrigerate until ready to serve; if preparing ahead, add avocado just before serving to prevent browning.