Make the Cilantro Ranch:In a blender or food processor, combine buttermilk, mayonnaise, sour cream, cilantro, jalapeño, ranch seasoning, and garlic. Blend until smooth. Taste and season with salt if needed. Chill until ready to use.
Marinate the Chicken:Toss the chicken thighs with BBQ sauce in a bowl. Cover and refrigerate for at least 30 minutes or up to overnight.
Cook the Chicken:Heat vegetable oil in a cast iron skillet over medium-high heat. Cook the chicken for about 5 minutes per side, until caramelized and fully cooked through.
Chop the Chicken:Transfer the chicken to a cutting board. Let it rest for 5 minutes, then cut into bite-sized chunks.
Assemble the Salad:In a large bowl, layer the chopped romaine, black beans, corn, cheese, tomatoes, crushed tortilla chips, shredded carrots, green onions, and chopped cilantro.
Dress and Serve:Drizzle the salad with cilantro ranch dressing and toss to combine. Top with BBQ chicken and serve immediately.