Generously season the brisket all over with salt and pepper. Place the brisket on a large rimmed baking sheet, cover with plastic wrap and refrigerate overnight.
Light a charcoal fire in a starter chimney. Add the lit coals to the firebox of a smoker and heat the smoker to 275°. Place oak or other hardwood chips, chunks or logs around the coals so that the wood smolders but does not flare. Set the brisket on the grill, fat side down. Cover and smoke for 2 hours. Monitor the smoker throughout the smoking process and add more lit coals and/or wood as needed to maintain the temperature and smoke level.
Meanwhile, in a small saucepan, combine the butter with the garlic and coriander and cook over moderate heat until fragrant, about 2 minutes. Add the barbecue sauce and beef broth and simmer the mop for 5 minutes. Season with salt and pepper.
After 2 hours, brush the brisket all over with the mop. Turn the brisket fat side up. Continue to cook, mopping every 30 minutes, until an instant-read thermometer in the thickest part registers 165°, about 6 1/2 hours longer.
Transfer the brisket to a large sheet of heavy-duty foil. Brush the remaining mop and garlic all over the brisket and wrap it in the foil. Put the wrapped brisket in a large, disposable aluminum roasting pan. Set the pan in the smoker and cook the brisket until it reaches 185°, about 1 hour longer.
Slice the brisket thinly across the grain and serve it with its cooking juices.