Trim the hard fat and silver skin from the brisket, leaving about ¼ inch of fat on the fat cap. Round off thin edges.
Brush the brisket with a thin layer of mustard or oil, if using. Season all sides evenly with the brisket rub.
Refrigerate uncovered for 12 to 24 hours.
Heat the smoker to 250°F. Let the brisket sit at room temperature for 30 minutes.
Smoke the brisket until the internal temperature reaches about 165°F and the bark is dark and set, about 6 to 8 hours. Spritz the edges every hour after the first 3 hours, if desired.
Wrap the brisket tightly in butcher paper or foil.
Continue smoking until the brisket reaches 200 to 205°F and is probe-tender in the thickest part of the flat, about 4 to 6 hours more.
Rest the wrapped brisket for at least 1 hour, or hold it in a towel-lined cooler for up to 3 hours.
Separate the point from the flat. Slice the flat against the grain into pencil-thick slices. Chop the point and toss it with warm BBQ sauce.
Serve on soft buns with pickles and onions, or on a platter with your favorite sides.