Place the brisket a baking sheet or a large cutting board. Trim the extra fat off of the brisket, leaving ~¼" of fat along the fat cap side. Toss the trimmings in the garbage. Pat the brisket dry with a paper towel.
Squirt with mustard Rub the mustard all over the brisket, on all sides and in all creases.
In a bowl, mix together the brown sugar, paprika, garlic powder, onion powder, cumin, salt, and pepper.
Sprinkle a liberal amount of the rub on top of the brisket.
Rub the mixture over the entire brisket, being sure to get it into the creases. Don't be shy with the spice mixture! Cover the brisket and let sit with the rub mixture in the refrigerator for 12-24 hours.
Add the pellets to your smoker. Preheat your grill to 225 degrees F. Add the brisket on a grate on the smoker and insert a meat thermometer probe into the thickest part of the meat. Close the lid and smoke to 165 degrees F, which will typically take about 8 hours.
At this point, lay out a long piece of butcher paper (or foil). Place the brisket on the paper and wrap the brisket, trying to fold the seams toward the bottom so the weight of the brisket keeps them sealed. I also had to use some tape.
Return the brisket to the smoker and keep cooking at 225 degrees F for 6-8 more hours, until the internal temperature reaches 205 degrees F. Note: there is always a period when the brisket temp will stall. I've had this happen anywhere between 165-185 degrees F. Don't worry - it will make it through this period. The cook time will vary, so be sure to use a meat thermometer!
Remove the wrapped brisket and let it rest for 30-60 minutes (or up to 4 hours if you wrap it and place it in a cooler!), depending on how much time you have. Cut into slices with a very sharp knife, slicing against the grain. Serve immediately. Enjoy!