Place the corned beef in a slow cooker and add beef broth, Guinness, brown sugar, black peppercorns, and bay leaves.
Cover and cook on LOW for 8 hours or until the meat is tender.
Remove the corned beef and let it rest before slicing.
In a skillet, heat butter and olive oil over medium heat.
Add the chopped cabbage and sauté until softened and slightly caramelized, about 10 minutes.
Stir in apple cider vinegar and season with salt and pepper.
Slice the corned beef and serve with caramelized cabbage and a drizzle of Guinness gravy from the slow cooker.