Trim the fat cap on the brisket flat down to about ¼ inch thick using a sharp knife. Remove any large hard pieces of fat and any tough silver skin from the meat side. Pat the entire brisket dry with paper towels, then return it to the refrigerator uncovered until you are ready to season and smoke it. Do not season the night before.
On the day of the cook, preheat your pellet smoker to 225°F using oak, hickory, or a competition beef blend pellet. While the smoker preheats, remove the brisket straight from the refrigerator and set it on a clean work surface. Sprinkle the Hardcore Carnivore Black seasoning directly from the bottle, holding it about 12 inches above the meat for even coverage. Aim for 75 to 80 percent coverage where you can still see meat through the rub, then press gently into the surface with your hands.
Set the brisket fat cap side up on the smoker grate with the thin end away from the fire pot. Insert a leave-in probe thermometer into the thickest part of the flat, making sure the probe tip is centered in the meat and not resting near the fat cap or the surface. Do not open the smoker until the meat reaches 110°F or the 2-hour mark, whichever comes first.
After 2 hours or once the brisket reaches 110°F internally, begin spritzing every 30 to 45 minutes. Hold the spray bottle about 12 inches above the surface and apply 8 to 10 pumps in an even pass across the full length of the flat, targeting the meat areas and avoiding the fatty areas when possible. The surface should look lightly glistened, not dripping.
Continue smoking and spritzing until the internal temperature reaches 160°F and the bark is dark mahogany in color and firm to the touch. This takes approximately 5 to 7 hours from the start of the cook.
When the internal temperature reaches 160°F, press a finger into the fatty areas. If the fat squishes and oozes, the brisket is ready to wrap. If not, increase smoker temperature to 250°F and continue cooking until the fat renders. If the internal temperature reaches 180°F before the fat renders, wrap it anyway.
Remove the brisket from the smoker and place it on a clean work surface. Lay a sheet of uncoated pink butcher paper on the surface, long enough to wrap the brisket on all sides with enough extra to tuck underneath. Pour all of the melted beef tallow onto the paper, spreading it across the area where the brisket will rest and the adjacent area where the paper will fold up over the top. Set the brisket meat side down into the tallow. Fold the near edge up and over by an inch or two, fold both side edges in snugly, then roll the package forward onto the remaining paper to cover the top. Roll forward once more to tighten and secure the ends. If using a leave-in probe, press it straight through the paper into the thickest part of the meat.
Return the wrapped brisket to the smoker fold side down. Cook at 250°F until the internal temperature reaches 200°F to 203°F and a probe slides into the thickest part of the flat with no resistance, about 2 to 3 more hours.
When the brisket is probe tender, remove it from the smoker. Leave it fully wrapped in the butcher paper and rest for a minimum of 2 full hours before slicing. For an extended rest, allow the internal temperature to drop to about 180°F on the counter, then place in a small cooler with a towel directly on top of the wrapped brisket. Close the cooler and allow the internal temperature to drop to 140 to 145°F, which can take 6 to 12 hours.
Unwrap the brisket and place it on a cutting board. Identify the direction of the grain — the long muscle fibers running through the flat — and slice against the grain in slices about ¼ inch thick. Slices should hold together but bend slightly when lifted. Serve immediately.