Stir together Coca-Cola, Worcestershire sauce, garlic, white vinegar, bouillon granules, chili powder, ground mustard, and cayenne.
Pat chuck roast dry. Sear in canola oil 2 to 3 minutes per side if desired.
Place roast in the slow cooker and pour marinade over and around the beef.
Cover and cook on low for 8 to 10 hours, until beef shreds easily with two forks.
Remove beef and shred, discarding any large fat pieces.
Return shredded beef to the slow cooker and toss with the cooking liquid.
Toast rolls with butter if desired, pile on beef, and serve with extra cooking liquid for dipping.
Notes
Use regular Coca-Cola because the sugar helps flavor the braising liquid. Searing is optional but adds browned flavor. Do not lift the slow cooker lid during cooking because it adds time. Use sturdy rolls so they hold the beef and dipping juice.