Yes, you read that correctly. They had this on the actual menu for one weekend and it sold like (pea)nuts. But, due to allergy concerns, they took it off the standard menu, but is available on the secret menu.
Ask them for peanut sauce on the side of your bacon Shackburger and you will drizzle that amazing sauce all over your bacon and patty.
WARNING: this may change your life and if you are a peanut butter fan, then you need to drive to your nearest Shake Shack now.