Set the brisket on a large sheet of plastic wrap. In a medium bowl, combine the dark brown sugar, chili power, paprika, salt, garlic powder, onion powder, black pepper, cayenne, dry mustard, and cumin, mixing to thoroughly combine. Rub the mixture onto the brisket and wrap tightly in plastic wrap. Place on a baking sheet and let marinate, refrigerated, at least 6 hours or overnight. Soak mesquite wood chips in a large bowl of water for 1 to 2 hours. Remove, drain and set aside.
Remove the meat from the refrigerator and let come to room temperature.
Prepare a smoker with charcoal and the wood chips according to the manufacturer’s instructions to approximately 200° to 225° F. Place the water pan in the smoker and add water to the fill line, about 2/3 full. Place the unwrapped brisket on the lower rack, off the direct heat. Close the lid and cook, regularly stoking the fire and adding additional chips, until an instant-read thermometer registers an internal temperature of 180° to 190° F, about 6 hours. Remove the meat from the grill and let rest for 20 minutes before carving the meat against the grain.
Alternatively, prepare a stove-top smoker according to the manufacturer’s instructions. Place the unwrapped brisket on the rack over low heat. Close the lid and smoke for 1 hour. Preheat the oven to 275° F. Remove the meat from the smoker and wrap in a large sheet of heavy aluminum foil. Place on a baking sheet and roast until tender and an instant-read thermometer registers an internal temperature of 180° to 190° degrees F, approximately 3 to 4 hours.
Remove the meat from the oven and let rest for 30-40 minutes before carving the meat against the grain.