Place a pot over high heat. Drizzle in vegetable oil. Add diced onion, ground beef, and salt. Break up meat with wooden spoon into small pieces as it browns. After meat browns and releases its juices, continue stirring until the released liquid evaporates, about 4 minutes. Reduce heat to medium-high. Add garlic, chile powder, cumin, paprika, black pepper, and cinnamon. Cook and stir until mixture begins to darken, 3 or 4 minutes.
Stir in beer. Add cocoa powder, oregano, cayenne, water, and tomato puree; stir well. Bring to a simmer; adjust heat to medium-low. Simmer 30 minutes.
Stir in diced green pepper and pinto beans. If mixture becomes too thick, add a bit more water. Simmer until peppers are tender and flavors have blended, about 30 more minutes.