Salt the meat: generously salt the meat at least one hour before adding the marinade. Cover and refrigerate in a large bowl. This step can be done a day in advance.
Clean pasilla chilies: wipe the dust off of each chile with a damp paper towel and remove the stem and seeds. If you like a little spice, leave some with seeds.
Rehydrate the pasilla chile peppers: Soak dried chiles in very hot water for 15 minutes or until soft. Reserve juice for later use.
Prepare the marinade: In a blender or bowl of a food processor, process the chiles, salt, oregano, garlic, lemon juice, and ginger on a high setting until you have a smooth chile puree. If necessary, add some of the liquid from the chile to get the blender going. The chile puree should be the consistency of barbeque sauce. Taste for salt. It should be on the salty side and no single flavor should stand out.
Marinate the beef: Place roast in a crockpot and coat well with sauce on both sides. Pour any leftover sauce over the meat. Add the reserved chile liquid, bay leaves, and vinegar. This step can be done a day in advance, especially if preparing the meat on the stove or in the oven. It is not necessary to marinate the meat as long when making it in a slow-cooker because of the extended length of time it cooks.
Slow cook the beef: There are two popular methods for slow-cooking barbacoa - in the oven or using a slow cooker. My preferred method is in the slow-cooker. For a hassle-free experience, cover the marinated meat with a lid and cook on low heat for 10 hours or until it reaches that fall-apart tenderness. Alternately, if using the oven method, preheat your oven to a low and slow 250°F (121°C). Place the marinated beef, including the marinade itself, in a roasting pan. Cover it tightly with tin foil and a lid if available, to create a very tight seal, and let it roast for about 5-6 hours or until the beef is incredibly tender and easily falls apart.
Shred the meat: Once the beef is tender, using a fork, shred it into bite-sized pieces. If there's any easily removable fat, it should be cut off at this stage.
Assemble and enjoy your tacos: Now comes the fun part—putting together your Barbacoa tacos. Warm your corn tortillas in a dry pan or oven until they're soft and pliable. Place a generous portion of shredded Barbacoa onto each warm corn tortilla. Garnish it with finely chopped onion, fresh cilantro leaves, a squeeze of fresh lime juice and your favorite salsa. My Spicy Golden Salsa and Pico de Gallo are two of my favorites! Serve alongside Classic Refried Beans, Mexican Red Rice, and fresh radishes topped with lime juice and salt.