Preheat a grill, grill pan, or skillet over medium-high heat.
Toss the chicken with 1/2 cup BBQ sauce in a bowl until evenly coated.
Grill the chicken, turning once, until lightly charred and cooked through, about 8–10 minutes; set aside.
Bring a large pot of salted water to a boil and cook the pasta until al dente according to package directions; drain.
Place the drained pasta in a large salad bowl and add the cubed cheddar, shredded romaine, halved cherry tomatoes, corn kernels, chopped jalapeño, and chopped cilantro and/or basil.
Make the ranch dressing: whisk together Greek yogurt (or sour cream), olive oil mayo, buttermilk, chives, dried parsley, dried dill, garlic powder, onion powder, and salt and pepper to taste in a bowl or jar.
Toss the pasta mixture with half of the ranch dressing, then add the grilled BBQ chicken and gently toss to combine.
Top the salad with diced avocado, season with additional salt and freshly cracked black pepper, and serve warm or cold with remaining ranch on the side.