Shredded Beef Taco Bar

Name: Shredded Beef Taco Bar
Ingredients: 5 dried guajillo chiles, stemmed and seeded; 5 dried ancho chiles, stemmed and seeded; 1/4 cup extra-virgin olive oil; One 4-pound bone-in beef shank, trimmed and tied by the butcher; Kosher salt; Pepper; 1 yellow onion, finely chopped; 5 garlic cloves, thinly sliced; 2 teaspoons dried oregano; 2 teaspoons ground cumin; Two 12-ounce bottles light Mexican beer; One 28-ounce can crushed tomatoes; 2 cups chicken stock or low-sodium broth; 2 tablespoons apple cider vinegar; 2 tablespoons unsalted butter