In a small bowl, make the sauce by whisking together the ketchup, dark brown sugar, molasses, distilled white vinegar, Worcestershire sauce, smoked paprika, chili powder, garlic powder, onion powder, dry ground mustard, cayenne pepper (if using), and tomato paste until well-combined; set aside.
In a medium to large skillet (I use a 10" skillet with 2" sides), heat the oil over medium heat. Add the onions and bell peppers; sauté, stirring occasionally, until softened, about 3 to 4 minutes.
Add the sauce and beans; stir to combine.
Increase the heat, bring to a boil, then reduce the heat a simmer gently, stirring occasionally, for 20 minutes. Taste and season with salt and pepper as needed.