Prepare smoker. Preheat your smoker to 225°F (110°C). Use wood chips like apple, cherry, or hickory for a sweet and smoky flavor.
Make rub. In a small bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, cumin, black pepper, kosher salt, and cayenne pepper (if using). Pat the pork tenderloins dry with paper towels and rub the olive oil over them. Coat the pork generously with the dry rub, pressing it into the meat.
Smoke. Place the pork tenderloins in the smoker and smoke for 1 to 1.5 hours, or until the internal temperature reaches 145°F (63°C) when checked with a meat thermometer.
Prepare glaze (optional). In a small saucepan, whisk together the honey, Dijon mustard, apple cider vinegar, soy sauce, and whiskey or bourbon (if using). Heat over medium-low, stirring occasionally, until the glaze thickens slightly, about 5-7 minutes.
Add glaze (optional). In the last 10 minutes of smoking, brush the pork tenderloins with the prepared glaze for a glossy finish. Do it gently so the dry rub stays on.
Rest. Remove the pork from the smoker and let it rest for 5-10 minutes before slicing. This helps keep the meat juicy.
Serve. Slice the tenderloin and serve with additional glaze on the side, if desired.