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In a large skillet over medium-high heat, brown the ground beef, breaking it up, until no pink remains (8-10 minutes). Drain most of the fat.
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In the same skillet, cook the diced onion for 3-4 minutes until softened. Add garlic and cook 30 seconds until fragrant.
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Transfer beef, onions, and garlic to the slow cooker. Add kidney beans, black beans, diced tomatoes (with juice), tomato paste, and beef broth. Stir to combine.
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Add chili powder, cumin, smoked paprika, salt, pepper, and brown sugar. Stir until spices are evenly distributed.
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Add the secret ingredients: chopped chipotle peppers with adobo sauce, cocoa powder, and yellow mustard. Stir well.
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Cover and cook on LOW for 7-8 hours, or HIGH for 3-4 hours. Low is better — the longer it cooks, the more flavors meld.
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About 30 minutes before serving, taste and adjust seasoning. Add more salt, cayenne for heat, broth if too thick, or leave lid off if too thin.
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Ladle into bowls and top with shredded cheddar, sour cream, diced onion, and serve with cornbread on the side.