Heat the oil in a 12 inch skillet over medium high heat for 2 minutes, then add the beef. Cook for 7 to 8 minutes, breaking it into small crumbles, until the beef reaches 160°F and the pieces look deeply browned, then drain all but 1 tablespoon of the fat.

Add the onion and pepper to the beef and cook for 5 minutes, stirring often, until soft and translucent. Stir in the garlic for 30 seconds, until fragrant.

Stir in the tomato paste, chili powder, and paprika, and cook for 1 to 2 minutes, until the paste darkens to brick red and smells sweet.

Whisk the ketchup, tomato sauce, broth, brown sugar, vinegar, Worcestershire sauce, mustard, salt, pepper, and optional pepper flakes together. Pour in the sauce mixture and stir to coat the beef. Bring to a gentle boil over medium heat, then reduce to a simmer.

Simmer uncovered for 12 to 15 minutes, stirring every 2 minutes, until the sauce is thick enough that a spoon dragged through the pan leaves a clear trail.

Spread the butter on the cut sides of the buns and toast them in a second skillet over medium heat for 1 to 2 minutes, until golden. Divide the filling evenly among the 6 bottom buns, top with cheese and pickles, and serve right away.