Preheat oven to 400 degrees F. Lay two long pieces of foil on a baking sheet – these will be used to fold up each tenderloin separately.
In a gallon-size freezer bag, mix salt with warm water until dissolved. Whisk in apple cider vinegar, brown sugar and ice, followed by pork. Brine for exactly 20 minutes then remove pork from brine, RINSE and and pat very dry. The pork becomes mealy if left in the brine any longer.
Whisk together all of the Spice Mix ingredients (not the "Add Later Herbs") in a small bowl. Evenly rub pork all over with spice mix.
Heat 2 tablespoons vegetable oil in a large skillet over medium high heat. When oil is very 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 pads 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) at the thickest part of the tenderloin, 15-25 minutes depending on thickness. This means the pork will be juicy and slightly pink in the middle. To check the temperature, carefully unwrap the foil, then wrap it back up if it need to continue baking.
When pork is done, loosen foil (so steam can escape) and let stand 5-10 minutes before slicing. Season with additional salt and pepper to taste (I like additional salt) and serve with accumulated butter sauce in the bottom of the foil.