Place two long pieces of foil on a baking sheet, set aside.
In a large mixing bowl, combine salt and brown sugar with warm water until dissolved. Once salt and sugar are dissolved, add apple cider vinegar and ice.
Place pork tenderloins in a large gallon zip lock bag and add the brine.
Brine pork for 20 minutes.
After 20 minutes, remove pork from the brine. Discard brine.
Preheat oven to 400 degrees F.
Rinse pork under cold water and pat dry with paper towels.
In a small bowl stir together chili powder, garlic powder, salt, onion powder, smoked paprika and pepper. Rub the spice mix onto each tenderloin, covering them with the spices.
In a large cast iron skillet, heat 1 tablespoon vegetable oil and 1 tablespoon butter over medium high heat. When oil is hot add a pork tenderloin. Sear each side of the pork tenderloin until golden brown. Transfer to one of the foil pieces on the baking sheet. Wipe out frying pan and repeat with second pork tenderloin.
Evenly sprinkle each tenderloin with the parsley, basil, oregano, and thyme and rub into pork.
For each tenderloin, drizzle lemon juice, then top each with pats of butter.
Bring up the sides of the foil to completely enclose each tenderloin.
Bake until pork reaches a temperature between 145 to 150 degrees F at the thickest part. This will take between 15-25 minutes depending on the thickness of each tenderloin.
Check the temperature after 15 minutes with an instant read thermometer by carefully unwrapping the foil. Rewrap and return to the oven if it needs to continue baking.
When pork has reached the correct temperature, open each foil pouch to allow steam to escape and wait 5-10 minutes before slicing.
Season with addtional salt and pepper if needed and serve with reserved lemon butter sauce.