Heat a large skillet over medium heat, add the beef stock, balsamic vinegar, Worcestershire sauce, and salt bring to a simmer, stirring frequently, reduce the stock by 2/3 to 1 ½ cups, which will take around 15-30 minutes depending on how large your skillet is. Once reduced, remove the reduction from the skillet and set aside.
Add butter, onions, celery, and carrots, sauté for 3 minutes until soft, then add garlic and sauté for 1 minute, then add the tomato paste and sauté for 1 minute. Add the molasses and beef broth reduction and return to a simmer for around 2 minutes
Combine cornstarch and bourbon to make a slurry. Whisking constantly, slowly add the slurry to the simmering sauce and whisk constantly until very thick, around 1-2 minutes
Remove from the heat, and add red pepper flakes and freshly cracked black pepper
Add the sauce to a food processor and puree it until smooth
Serve the sauce steaming hot or store the sauce for 1 week in a tightly sealed container, reheat before serving