Heat olive oil in a large skillet over high heat. Add pork butt and cook about 2 minutes per side, until nicely seared.
Place pork in a crockpot and top with onions.
In a small bowl, combine chili powder, salt, brown sugar, cumin, oregano, black pepper, cayenne, garlic powder, and cinnamon. Sprinkle on top of pork. Add chipotle pepper, bay leaves, and tomato paste to crockpot. Don't worry about mixing tomato paste in, just smear it on top of pork. Cover crockpot and cook on low for 8 hours.
Remove meat to a large bowl and shred. Remove 2 cups for the quesadillas and reserve the remaining pork for another use.
Add barbecue sauce to the 2 cups of pork and mix well.
In a small bowl, mix avocado, sour cream, and lime together, using a fork to mash the avocado. Set aside.
Heat a small amount of vegetable oil in a large nonstick skillet set over medium heat. For each quesadilla, use about ½ cup pork, ¼ cheese, and ¼ of the sliced mango. One quesadilla at a time, cook in the skillet for 1-2 minutes per side, until lightly browned and cheese is melted.