Shake Shack's Shack Stack combines a ShackBurger beef patty with a Shroom Burger patty on the same bun, topped with lettuce, tomato, and ShackSauce. The ultimate indulgence on Shake Shack's menu, it gives you the best of both worlds — beef and cheese-stuffed mushroom — in one sandwich and is the go-to order for regulars who can't choose between the two.