Method
1. Sear the Beef
Pat the beef dry and season generously on all sides with kosher salt and black pepper. Heat the oil in a large skillet or Dutch oven over high heat until shimmering. Working in batches to avoid crowding the pan, sear the beef until deeply browned on all sides, about 3-4 minutes per side. Transfer to the slow cooker.
2. Build the Aromatic Base
Reduce the heat to medium. Add the diced onion to the same pan and cook until softened, about 4-5 minutes. Stir in the garlic, chipotles with their adobo, and tomato paste. Cook for 2 minutes, until the paste darkens to a brick red. Add the cumin, ancho, smoked paprika, and oregano and toast for 30 seconds, just until fragrant.
3. Deglaze the Pan
Pour in the Coca-Cola and beer, scraping up all the browned bits from the bottom of the pan. Let bubble for 1 minute, then pour everything into the slow cooker over the beef.
4. Slow Cook
Add the beef broth, brown sugar, green chiles, enchilada sauce, and cinnamon stick to the slow cooker. Stir to combine. Cover and cook on LOW for 8-9 hours (or HIGH for 5, if pressed for time). The beef is done when it shreds easily with a fork.
5. Shred and Reduce
Transfer the beef to a bowl. Discard the cinnamon stick. Strain the braising liquid into a saucepan, skim off the surface fat, and simmer over medium-high heat for 10-15 minutes, until reduced by about a third and slightly syrupy—it should coat the back of a spoon. While the liquid reduces, shred the beef with two forks.
6. Finish and Rest
Off the heat, stir the dark chocolate into the reduced sauce until melted. Add the lime juice and the reserved tablespoon of adobo sauce. Taste and adjust for salt. Return the shredded beef to the slow cooker, pour the sauce over the top, and stir to combine. Let it rest on the WARM setting for at least 20 minutes so the meat reabsorbs the sauce. Stir in the chopped cilantro just before serving.