Chef royalty and celebs alike proclaim their love for In-N-Out which begs the question, what’s its secret? I mainly came to In-N-Out for their chocolate shakes, fries and occasionally a burger with cheese. It may just be one extra patty from my usual order, but OH did it make a difference. The key difference is that they cook this patty with mustard on the grill, giving it an extra tangy flavor. This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses.