Layer the chicken breast between sheets of plastic wrap. Pound with a meat mallet or rolling pin to an even thickness of about 1/2 inch.
In a small bowl combine oil, spices, adobo sauce and lime juice. Finely chop canned chipotle pepper and add to spice mixture. Stir mixture to create a paste and rub onto both sides of the chicken breasts. Cover, refrigerate until ready to use. Can be done up to 6 hours ahead.
When ready to assemble tacos, heat grill to medium high and cook chicken breast for 6-8 minutes per side, depending on how thin you pounded them. Remove from grill, cover with tin foil and allow to rest for 5 minutes before slicing.
Cut avocados in half and remove the stone. Scoop out flesh into a food processor, mini chopper or blender. Add all remaining ingredients except cilantro and blend until desired consistency. Stir in chopped cilantro, adjust salt seasoning as desired.