Place your brisket on a large cutting board, fat-cap side up. Trim off the fat cap, as most of it is hard fat that doesn't render and prevents the meat from absorbing the smoke. However, you'll want to leave a narrow layer of the softer fat (about ¼ inch thick) on the outside to keep it from drying out.
A fairly thick vein of fat runs between the point and the flat, but you won't want to remove it because it keeps the brisket moist. You can easily remove it while slicing the cooked brisket.
If the rub is salt-free, salt the brisket with about ½ teaspoon per pound. If the rub has salt, then half the amount. Rub the roast with yellow mustard (or Worcestershire sauce) so the spices stick better. Thoroughly coat the brisket with the dry rub. At this point, you can chill it in the fridge (uncovered on a wire rack over a baking sheet) for 12-24 hours for the salt and seasonings to do their job and form a good crust.
Preheat your pellet grill to 225-250°F (110-120℃), per the manufacturer's instructions.
Take the brisket out of the fridge and put it directly in the pellet grill, fat-side up. Smoke it for about 6 hours, or until the internal temperature reaches 165-170℉ (75-80℃) and you have a nice crusty bark.
Remove it from the smoker and wrap it tightly with butcher paper or foil. (Foil cooks faster, but it might ruin the bark. However, butcher paper preserves the texture of the bark but takes longer.)
Return the wrapped brisket to the smoker and continue cooking until the internal temperature is between 200-205°F (95-98℃), 6-7 hours. You can also do this in a 225-250°F (110-120℃) oven.
Place the wrapped brisket on the countertop for 10-20 minutes so it releases steam, then put it in a Styrofoam cooler or a 145-170°F (65-80℃) oven for 2-3 hours. (See notes)
Unwrap the brisket, slice it against the grain, and serve with your favorite barbecue sauce and sides.