Combine oil, lime juice, brown sugar and spices in a small bowl. Pound chicken breasts to an even thickness and pierce with fork or meat tenderizer. Place chicken and marinade in a plastic bag and refrigerate overnight.
Make Chipotle Cream Sauce.
Grill chicken until internal temperature reaches 165 degrees.
Assemble tacos with Chipotle Cream Sauce, onion, cilantro, cheese and avocado.