Add buttermilk, mayonnaise, sour cream, cilantro, jalapeño, ranch seasoning, and garlic to a blender. Blend until completely smooth and creamy. Taste and adjust seasoning if needed, then refrigerate while you prepare the rest.
Coat the chicken thighs thoroughly with BBQ sauce. Let them marinate in the refrigerator for at least 30 minutes if you have time. This step deepens the smoky flavor.
Heat vegetable oil in a skillet over medium heat. Cook the chicken about 5 minutes per side until fully cooked and caramelized. Remove from heat and let it rest for several minutes before chopping.
Once slightly cooled, chop the chicken into bite-sized pieces. Cutting it small ensures every forkful gets a perfect balance of flavors.
In a large bowl, combine romaine lettuce, black beans, corn, carrots, tomatoes, green onions, cilantro, and shredded cheese. Toss gently so everything distributes evenly.
Pour cilantro ranch over the salad and toss lightly to coat. Top with chopped BBQ chicken and crushed tortilla chips. Serve immediately for the best texture and flavor.