Heat olive oil in a large pot over medium heat; cook and stir onion until softened, 5 to 10 minutes. Add garlic; cook and stir until fragrant, about 1 minute. Stir beef, green bell pepper, chili powder, cumin, celery seed, basil, chile pepper, bay leaves, and salt into onion mixture; cook and stir until beef is no longer red in the center, about 10 minutes.
Stir tomatoes into beef mixture, decrease heat to low, and simmer for 4 hours.
Stir kidney beans and cocoa powder into the chili; cook, stirring occasionally, for 1 hour more.