In 3 1/2- to 4-quart slow cooker, combine all ingredients except beef and buns. Add beef; mix well.
Cover; cook on high setting for 4 hours or on low setting for at least 8 hours or until beef is tender.
Remove beef from slow cooker; place on large plate. Shred beef; return to slow cooker and mix well.
To serve, spoon 1/3 to 1/2 cup beef mixture into each bun. If desired, top with shredded cheese or sour cream.