1Place chilies in large heat-safe bowl. Add boiling water; cover and let stand 15 minutes until softened. Remove chilies with tongs and place in blender container. (Discard soaking liquid.) Add remaining marinade ingredients, cover. Blend until smooth.
2Place beef in large resealable bag or glass bowl. Add marinade; turn to coat well. Refrigerate at least 2 hours or overnight.
3Preheat oven to 350°F. Place beef and marinade, beef stock and bay leaves in large oven-proof saucepan or Dutch oven. Cover pan tightly with foil, then cover with lid. Transfer to oven.
4Braise 4 hours or until meat is fork tender. Remove meat with a slotted spoon into a large bowl, reserving braising liquid. Shred meat with two forks. Stir in 1 cup of the reserved braising liquid. Place remaining braising liquid in a serving bowl for dipping.
5Serve meat with warm tortillas, chopped cilantro and white onion, and crumbled Cotija cheese, if desired, along with braising liquid for dipping.
6To prepare in a Slow Cooker, follow instructions through Step 2. Place beef and marinade and bay leaves in large slow cooker. Cover. Cook 6 hours on HIGH or 10 hours on LOW. Shred meat and serve as directed.