Step 1: Prepare the Spice Rub. In a small bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, salt, and black pepper. Mix well until the spices are evenly distributed. Pat the brisket dry with paper towels and rub the spice mixture all over the meat, pressing it in so it adheres well.
Step 2: Slow Cooking the Beef. Place the seasoned brisket into your slow cooker. In a measuring cup, whisk together the beef broth, liquid smoke, and half of the BBQ sauce. Pour this liquid around the sides of the brisket (avoid pouring directly over the spice rub to keep the crust intact). Cover and cook on LOW for 8 to 10 hours or until the beef is fork-tender.
Step 3: Shredding and Saucing. Once the beef is fully cooked, remove it from the slow cooker and place it on a large cutting board or rimmed baking sheet. Use two forks to shred the meat into bite-sized pieces. Discard any large chunks of fat. Return the shredded beef to the slow cooker or a bowl and toss with the remaining BBQ sauce and a splash of the cooking juices.
Step 4: Assembling the Sandwiches. Split the buns and lightly toast them under a broiler or in a pan with a little butter. Pile a generous amount of the smoky shredded brisket onto the bottom half of each bun. Top with pickles, coleslaw, or extra sauce if desired, and place the top bun on to finish.