01 - Pat the brisket thoroughly dry with paper towels. Combine kosher salt, black pepper, smoked paprika, garlic powder, onion powder, and cayenne pepper in a small bowl. Massage the spice rub evenly over all surfaces of the meat, pressing gently to adhere.
02 - Heat olive oil in a large skillet over medium-high heat until shimmering. Sear the brisket on all sides until deeply browned, approximately 2 to 3 minutes per side. Use tongs to turn the meat carefully.
03 - Place the seared brisket into the slow cooker insert, fat-side up.
04 - Whisk together ketchup, apple cider vinegar, brown sugar, Worcestershire sauce, Dijon mustard, smoked paprika, garlic powder, onion powder, black pepper, and sea salt in a mixing bowl until completely smooth and the sugar has dissolved.
05 - Pour half of the barbecue sauce mixture evenly over the brisket. Reserve the remaining sauce in a sealed container for serving.
06 - Cover the slow cooker with its lid. Cook on LOW setting for 8 hours or on HIGH setting for 4 to 5 hours. The brisket is done when it shreds easily with minimal fork pressure and reaches an internal temperature of 205°F.
07 - Carefully transfer the cooked brisket to a cutting board. Allow the meat to rest for 10 minutes before slicing against the grain or shredding with two forks.
08 - Skim any excess fat from the surface of the sauce remaining in the slow cooker. Return the sliced or shredded brisket to the slow cooker and toss gently to coat thoroughly in the cooking juices.
09 - Transfer the sauced brisket to a serving platter. Serve immediately with the reserved barbecue sauce on the side for guests to add additional portions as desired.