Pat brisket completely dry with paper towels. Coat all sides with mustard - this helps the rub stick and adds no flavor. Mix salt, pepper, smoked paprika, garlic powder, onion powder, and cayenne in a small bowl. Rub generously over entire surface, pressing into any crevices. Let sit at room temperature for 30 minutes while oven preheats.

Position oven rack in lower third and preheat to 300°F. Set wire rack inside rimmed baking sheet. Place brisket fat side up on rack. Roast uncovered for 3 hours until a dark crust (bark) forms and internal temperature reaches about 160°F. The exterior should look deeply browned and slightly crusty.

Remove brisket from oven. Mix beef broth with 1 tablespoon liquid smoke. Lay out two long sheets of heavy-duty foil, overlapping. Place brisket in center, pour broth mixture around (not over) the meat. Add remaining 1 tablespoon liquid smoke drizzled on top. Wrap tightly, crimping edges to seal completely. Return to oven, still at 300°F.

Continue roasting wrapped for 2 to 3 more hours, until internal temperature reaches 200-205°F and a probe or skewer slides in with little resistance like butter. This is more important than temperature - the connective tissue must fully break down. Total cook time will be 5 to 6 hours depending on thickness.

Transfer wrapped brisket to a cutting board, still sealed in foil. Let rest for at least 30 minutes, up to 1 hour. This allows juices to redistribute. Unwrap, reserving any juices in the foil to pour over sliced meat. Slice against the grain into 1/4-inch thick slices. Serve immediately with your favorite BBQ sauce on the side.