If you don't already know, the Shake Shake burger is practically a religion for some New Yorkers. One blogger at Serious Eats reverse engineered the burger, including the all important special sauce.
I walked back into the restaurant, went straight up to the manager, and asked point blank: "Is the Shack Sauce a secret, or can you tell me what's in it?" A little laugh, and then, "It's mostly mayo, with some ketchup, mustard, a few spices, and pickles blended in."