Shake Shack Sauce
from J. Kenji López-Alt, https://www.seriouseats.com/

After trying several ways to find out what goes into the Shake Shack burger sauce, J. Kenji López-Alt walked up to the manager and asked him. "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." "So, pickle relish, or pickles?" "Actual pickles—the sliced pickles we serve with the burgers. I couldn't give you exact tablespoon measure or anything because I don't know them off hand, but that's the general idea."

He then went home with some extra Shack Sauce and came up with this recipe.

1/2 cup (120ml) mayonnaise
1 tablespoon (15ml) ketchup
1 tablespoon (15ml) yellow mustard
4 slices kosher dill pickle, finely chopped
1/4 teaspoon garlic powder
1/4 teaspoon paprika
Pinch cayenne pepper

Combine all ingredients in a countertop blender jar and blend on high until smooth, scraping down sides of blender with rubber spatula as necessary, 30 seconds to 1 minute. Transfer sauce to an airtight container and refrigerate until ready to use (sauce can be refrigerated for up to 5 days).

Makes 3/4 cup

To make the burger, butter a hamburg bun keeping the top and bottom bun still hinged together and toast the buttered side of the roll under the broiler or a toaster oven. Spread the top half with the Shake Shack Sauce, then place 1 leaf lettuce and 2 slices tomato on top half of each bun. Fry the hamburgers by pressing each down flat in the pan. Cook until crisp brown crust has formed on the bottom. Flip the burger and top with 1 slice of American cheese. Cook until cheese is melted, about 1 minute longer. Transfer patties to burger bun bottoms, and close sandwiches.

To be authentic, slip into a wax-paper sleeve (or a jury-rigged parchment paper sleeve), wait 30 seconds for steam from patty to penetrate and soften bun, then consume. A perfect taste-alike.