Preheat the oven to 400 degrees F and move the rack to the middle position. Line a large baking sheet with tin foil then lay another two long pieces on top on the prepared baking sheet – these two pieces will be used to fold up each tenderloin separately.
Prep the tenderloins by cutting off any excess fat and the silver skin (the tough part that may cover some of each tenderloin).
In a large freezer bag, mix salt with warm water until dissolved. Whisk in brown sugar, apple cider vineage and ice, followed by pork. Brine for 30 minutes then remove pork from brine, rinse and pat very dry.
Whisk together all of the Spice Mix ingredients (not the “Herbs To Add Later”) in a small bowl. Evenly rub spice mix all over / on both sides of the pork tenderloins.
Heat 2 tablespoons extra virgin olive oil in a large skillet over medium high heat. When oil is hot and just smoking, add pork tenderloins, you may need to work in batches. Sear each side of pork until golden, then transfer to foil.
Evenly sprinkle tops of pork tenderloins with remaining herbs (parsley, basil, oregano, thyme) then rub into pork.
Evenly drizzle each pork tenderloin with one tablespoon of lemon juice (2 tablespoons total), then top each tenderloin with 3 pats of butter. Bring up the sides of the foil to completely enclose each tenderloin.
Bake at 400 degrees F until pork registers between 145- and 150-degrees F (63 and 65 degrees C) on a meat thermometer at the thickest part of the tenderloin, 25 minutes depending on thickness. To check the temperature, carefully unwrap the foil, then wrap it back up if it needs to continue baking.
When pork has come to temperature, loosen foil (so steam can escape) and let stand 5-10 minutes before slicing to lock in juices.
Slice the pork across the grain ½ inch slices, and spoon all of the delicious buttery juices from the bottom of the foil ontop of the pork when serving. Serve immediately.