Combine all spices and seasonings in a small bowl to create your dry rub mixture. Mix together the sugar, paprika, salt, pepper, chili powder, and cumin until evenly distributed. This pre-mixed rub ensures the spices distribute evenly across the brisket and prevents clumping when it hits the moist environment of the slow cooker.
Rub the entire surface of the brisket generously with the dry rub mixture from Step 1, working the spices into all crevices and the fat cap. Place the seasoned brisket into your slow cooker. Pour the Worcestershire sauce and apple cider vinegar directly over the meat, then spread the BBQ sauce evenly on top. I like to reserve about ¼ cup of the BBQ sauce to stir in at the end—this keeps the sauce fresher tasting and prevents it from becoming too concentrated after hours of cooking.
Cover the slow cooker and cook on low heat for 8-10 hours, or on high heat for 4-5 hours if you're short on time. The meat is done when a fork easily pierces the thickest part and the brisket shreds with minimal resistance. Low and slow cooking breaks down the connective tissue in the brisket, transforming a tough cut into tender, pullable meat. Plan your timing so the brisket finishes about 15-20 minutes before you plan to serve—this gives you time to shred and finish the sauce.
Transfer the cooked brisket to a large cutting board using tongs or forks, leaving the liquid in the slow cooker. Using two forks, pull the meat apart into shreds, discarding any large pockets of fat as you go. Return the shredded beef to the slow cooker and stir thoroughly to combine with the sauce. Taste and adjust seasoning if needed—you can always add a splash more Worcestershire or apple cider vinegar if it needs more punch. I find the sauce often needs a bit more acidity after hours of cooking, so don't skip this tasting step!
Lightly toast the brioche buns if desired for better texture and to prevent them from getting soggy. Generously fill each bun with the shredded beef and sauce mixture using a slotted spoon or tongs to allow excess liquid to drain back into the slow cooker. Serve immediately while the beef is still warm, optionally adding coleslaw, pickles, or additional BBQ sauce on the side.