Preheat the oven to 400°F, with the rack in the middle position.
Make the spice rub. In a small bowl, stir together the brown sugar, smoked paprika, garlic powder, onion powder, chili powder, Italian seasoning, salt, and pepper. Coat the tenderloins all over with the rub.
Set up the pan. Place the tenderloins in a 9×13-inch (or similarly sized) baking dish, and scatter the butter pats over the top.
Bake, uncovered, for about 25 minutes, or until the pork reaches an internal temperature of 145°F. Let it rest 5-10 minutes before slicing into medallions, this keeps all that juice inside instead of on your cutting board.
Finish it off. Pour the pan juices over the sliced pork (they're also excellent spooned over mashed potatoes), and sprinkle with chopped parsley if you like.
