Place olive oil, brown sugar, lime or lemon juice, garlic powder, salt, onion powder, dark chili powder, dry oregano, ground chipotle or ancho pepper, ground mustard, paprika and cocoa powder into a small mixing bowl. Mix to form to paste.
Rub on all sides of the pork tenderloin then cover with plastic wrap and place into the refrigerator. Marinate the tenderloin at least 6 hours or overnight.
Prepare: Remove from the fridge around 20 minutes prior to grilling to allow the meat to warm to room temperature.
Oil the grill grates and heat to medium-high. Seer the tenderloin over high heat for 2 minutes on all 4 sides. (You can also do this indoors on grill pan.)
Lower heat to medium and continue to cook on the widest side for 6-8 minutes each depending on the thickness of the tenderloin. Adjust the cooking time as needed based on thickness. The tenderloin pictured were approximately 2 1/2-3 inches at their thickest. The general recommendation is to cook whole pork [not ground] to an internal temperature of at least 145°F or to medium rare.
Baste with the grilling sauce during the last 3-5 minutes of grilling to char.
Rest loosely covered with aluminum foil for about 5-10 minutes before carving.