Trim any excess fat along the top of the meat of the brisket. I like to keep the cap of fat on the brisket. I do not trim it because it helps add flavor to the beef.
If necessary, separate the point from the flat. Illustrative details can be found here.
Fully coat the brisket with the rub, salt, and pepper to taste. Place the brisket in the fridge to absorb the flavor of the spices. Minimum 30 minutes, but a few hours to overnight is best.
Preheat smoker to 175 degrees. I typically cook it overnight and start at 175 degrees to avoid overcooking it. You can also smoke it at 225 degrees.
Add the brisket to the smoker, fat side down. Smoke until it reaches an internal temperature of 175 degrees.
Remove the brisket from the smoker and drizzle it in the butter and apple cider vinegar. Wrap it tightly in butcher paper or foil. Be sure it's tight so that the moisture is locked in.
Add the wrapped brisket to the smoker at 250 degrees fat side up. This will allow the fat from the brisket to seep into the beef as it smokes. Cook until it reaches an internal temperature of 203-205 degrees.
Remove the brisket and allow the brisket to rest inside the wrap for at least 30 minutes. I allowed it to rest for over an hour before unwrapping and slicing.