In a small bowl, combine apple butter, brown sugar, apple cider vinegar, molasses, garlic, and sriracha. Whisk gently with a fork. Set about 1/3 of the mixture aside.

Heat grill on a medium setting. While grill is preheating, trim extra fat from pork tenderloin. Place in a large glass baking dish (i.e. a large rectangular Pyrex) and sprinkle with kosher salt and freshly ground pepper. Keeping 1/3 of the sauce on the side, brush pork tenderloin with some of the remaining.

Place pork on hot grill. Grill for 10 minutes, apply more sauce and flip. Apply additional sauce as needed, cooking until the pork reaches 150F in the thickest part.

Remove pork tenderloin from grill and transfer to a clean Pyrex or cutting board. Allow to rest for 5 minutes, covered with foil.

With a clean brush, brush pork with remaining sauce. Sprinkle with chives. Slice and serve hot.