In a blender, add all of your ingredients except the butter, broth, bay leaves, diced onions, tomato paste, roast (obviously), and the tortillas & cheese.
You've now made your barbacoa sauce.
In a large dutch oven pot, heat your butter on medium heat. Once it foams, add your onions. Allow them to brown and then add your chuck roast. Brown the roast on all sides. Once the beef is browned on all sides, add your barbacoa sauce by pouring it over the beef. Then add your chicken broth and your bay leaves. Cook on low heat (stirring intermittently) for four to five hours, this will help develop a rich flavor for your dish.
After about four or five hours, you should have a rich sauce and beef that comes apart easily with just a fork. If your sauce is too watery, slowly reduce the sauce so it thickens. Pull the beef apart in the sauce, this way the beef with soak up all the yummy flavors from the sauce.
Your barbacoa is now ready to eat, serve it with some warm tortillas and chow down!