Rub the pork shoulder on all sides with the minced garlic, massaging it into the meat.
Combine the salt, cumin, chili powder, black pepper, oregano, cayenne pepper and cocoa powder in a small bowl then sprinkle the spice mixture evenly over the meat. Transfer the pork shoulder to the slow cooker then add the lime juice, orange juice, salsa and beer.
Cover the slow cooker and cook on "low" for 8 hours, or on "medium" for 6 hours.
Remove the pork from the slow cooker (leaving behind any liquid), shred and serve in corn tortillas with assorted toppings.
You can opt for a Spanish-style beer, such as Negra Modelo or Corona, or just use whatever beer you have on hand.