Prepare the Pork: Gently pat the pork tenderloins dry with paper towels. Carefully trim any visible excess fat or the tough, shiny "silver skin" membrane from the surface of each tenderloin for the most tender result.
Combine Rub Ingredients: In a small mixing bowl, thoroughly combine the packed brown sugar, smoked paprika, garlic powder, onion powder, chili powder, Italian seasoning, salt, and ground black pepper.
Season the Pork: Place a tenderloin on a clean plate or cutting board. Generously coat all sides of the tenderloin with the spice rub mixture, pressing lightly to help it adhere. Repeat with the remaining tenderloins. Reserve any leftover rub.
Arrange in Slow Cooker: Pour the beef broth and Worcestershire sauce into the bottom of your slow cooker. Carefully place the seasoned pork tenderloins into the slow cooker. Sprinkle any remaining spice rub from the plate over the pork. Distribute the pieces of butter on top of and around the tenderloins.
Cook: Secure the lid on the slow cooker. Cook on the Low setting for 2.5 to 3.5 hours, or on the High setting for 1.5 to 2.5 hours. Cooking times can vary significantly between slow cooker models and depending on the exact size and thickness of your tenderloins.
Check for Doneness: Begin checking for doneness towards the lower end of the cooking time range. Insert an instant-read meat thermometer into the thickest part of each tenderloin. The pork is ready when it reaches an internal temperature of 145∘F to 150∘F (63∘C to 66∘C). Avoid overcooking, as tenderloin can become dry. If not yet at temperature, replace the lid and continue cooking, checking every 10-15 minutes.
Rest and Serve: Once cooked, carefully remove the pork tenderloins from the slow cooker and transfer them to a clean cutting board. Tent them loosely with foil and let them rest for 5-10 minutes. This allows the juices to redistribute, ensuring a more tender and flavorful result. After resting, slice the tenderloins into ½-inch to ¾-inch thick medallions. Serve the medallions drizzled with the pan sauce from the slow cooker.