Prepare the chiles by removing the stems and seeds. Roast the chiles in a dry heavy bottom pan like an iron skillet over medium high heat.
Once the chiles are roasted and cooled, briefly cut or tear them into smaller pieces and process them in a spice grinder. Working in batches, sieve the chiles with a fine mesh strainer to remove anything that wasn't completely ground.
Roast and grind the cumin seeds. This will extract the full flavor from the cumin. Sieve the cumin seeds as you did the chiles and discard anything that didn't get ground up. (Alternatively, use pre-ground cumin to save time.)
Once all of the chiles/cumin have been ground, add the dry spices (ancho, morita, oregano, cocoa powder, cumin, espresso powder [optional], cinnamon, sea salt and coarse black pepper) to a bowl and mix thoroughly to combine.
Rub the short rib meat thoroughly with the spice mixture to coat evenly. Cover and marinate the meat in the spices overnight. If you don't have time to marinate, you can still use the meat right away, the flavors are just stronger if marinated.
Using a slow cooker, add the seasoned beef short ribs to the container and top with the sliced onion, minced garlic and fresh orange juice. Place the lid on the container and slow cook the meat according to the directions on your slow cooker (usually on low for about 6-8 hours or until fork tender).Alternatively, you can place the meat in a heavy bottom pot with a tight fitting lid on low heat, but this route will cook the meat much faster (in about 2 hours until tender). Once the meat is fork tender, strain with a colander fitted over another pot to save the sauce. If the sauce is very liquidy, you can reduce it and then add back into the shredded meat. Or place the sauce in the fridge for a couple of hours to solidify any fat and remove it from the sauce once it has congealed. Then add the sauce back to the meat.
Now that the meat is fork tender and the sauce has cleaned of any excess fat, the Mexican style beef short rib is ready to used in any way you wish. The meat can be frozen and will keep well for several months. When I make this I will double or triple the amount so I can have it ready whenever I need it for making a variety of different Mexican style dishes.