In a large saucepan heat the oil over medium-high heat. Add sweet pepper and onion; cook and stir for 4 to 5 minutes or until tender. Add beans and chili powder; cook and stir 2 minutes. Stir in tomato sauce, undrained tomatoes, brown sugar, and cocoa powder. Bring to boiling; reduce heat. Cover and simmer 25 minutes, stirring occasionally.
Top with sour cream, green onions, and additional chili powder, if desired.