Whisk together vegetable oil, salt, chili powder, paprika, cumin, cayenne pepper, minced garlic, brown sugar, and chipotle pepper.
In a gallon sized bag combine marinade and chicken and refrigerate for 30-60 minutes.
Once ready to grill, thread the chicken on skewers. Place them on the grill turning once or twice until sides are cooked and insides are no longer pink. Place on a plate and sprinkle with fresh cilantro if desired.
In a food processor combine avocado, greek yogurt, garlic, lime juice and cilantro. Pulse until smooth. Add salt and pepper to taste and serve with chicken.