In a small bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, chili powder, Italian seasoning, salt, and red pepper flakes. Stir well so the seasoning is evenly mixed.
Place the pork tenderloins on a cutting board. Sprinkle the seasoning mixture evenly over all sides of the pork, gently pressing it in so it adheres well.
Transfer the seasoned pork tenderloins to the crock pot, laying them flat. Pour the beef broth around the pork, then slice the butter into small pats and place them over the top.
Cover and cook on Low for 6 to 7 hours or on High for 3 to 4 hours. Begin checking the internal temperature after about 1½ hours. The pork is done when it reaches 145 to 150 degrees.
Once cooked, carefully remove the pork tenderloins from the slow cooker and let them rest on a cutting board for about 30 minutes. This helps keep the meat juicy.
Slice the pork into thick slices, then return it to the crock pot so it can soak up some of the buttery broth before serving.
