What makes Shake Shack's new black truffle sauce so good

Shake Shack is adding an upscale special ingredient to its beloved fast food menu: black truffle.

The burger chain has tapped into its fine dining roots in partnership with rare and specialty importer Regalis Foods to create three elevated new menu items topped with a one-of-a-kind organic black truffle sauce.

"What we wanted to do is really harness those fall, wintery flavors and also bring more ingredients to the table that really pair with black truffle," Shake Shack culinary director Mark Rosati told ABC News of their long-anticipated collaboration with Ian Purkayastha, the Regalis' founder who works directly with chefs in 90% of the Michelin-starred U.S. restaurants.

"Through COVID, as a brand we're constantly trying to pivot with everyone in the industry -- so we think this is an awesome opportunity for us," Purkayastha said of the ability to collaborate on a high-end experience at Shake Shack.

First up, Rosati concocted a new cheeseburger made with Gruyère cheese, which he explained has "a lot of harmony with the earthiness of the black truffle."

"For the sauce, we wanted to do something that had a little more complexity" to make the truffle flavor even more pronounced.

"We caramelized mushrooms, chopped them up finely with a little bit of garlic and shallot" and that mixture cooks down with sherry vinegar, madeira and port wine to create a reduction. "Then it gets folded into a mayonaisse and added the black truffle oil from Ian [Purkayastha]," Rosati said of the spread made with the top of the line foraged truffle-infused oil.

Rosati said "it needed a textural component" so he topped it with crispy shallots to lend a crunch and "garlicky punch -- that goes well with the truffles."

The second new product that will feature the specialty, seasonal sauce is a cheese-filled, crispy 'shroom burger -- which in Rosati's professional opinion is "the sleeper" of the menu.

"We're putting the truffle sauce on the top and bottom bun, usually we just do it on one side -- and a little bit of shredded lettuce -- when you bite into it with the cheese oozing out and it mixes with the truffle sauce, it's next level," Rosati said.

Third, is a stripped down version of the Shack Stack, a combination of their burger patty and crispy mushroom.

Rosati said it has "a double spread of the black truffle sauce, a Gruyère cheeseburger and a 'shroom on top -- We're trying to really do right by the truffle flavors and make the whole thing a cohesive delicious experience. One of my favorite things is also putting the sauce on the fries."

Purkayastha told ABC News "it's been very exciting to watch" Rosati develop a menu featuring his USDA Organic Black Truffle Arbequina Oil "at an approachable price point."

Pricing for the Black Truffle Burger is $8.89, a Black Truffle ‘Shroom Burger will cost $8.89, the Black Truffle Shack Stack is $11.89 and a side of the Black Truffle Sauce is just $1.50.

"Truffles are kind of the last entirely natural and wild food left in existence that's never been successfully cultivated on a commercial scale," he explained. "Coupled with their extreme perishability" and the fact that it takes a trained dog to sniff out the crop "makes them expensive."

Nearly 98% of truffle products sold globally are artificially flavored with synthetic lab-created essences and aromas, which can be cloying and off-putting to some people's palates; Regalis’ oil is flavored organically with real truffles and is the only truly natural truffle product on the market.

"We've always wanted to find ways to make truffles a little bit more mainstream and at the same time preserving the quality that we've worked hard to establish," Purkayastha said. "We're the only company that's been able to craft a product made with real truffles at a price point that's now available for the masses. Working with Shake Shack we've been able to crack the code and make it happen."

"Ian has this amazing product -- and we're all about the best ingredients we can possibly work with and this raises the bar to a whole other level," Rosati said.

The menu is available from Dec. 1 through Dec. 31 while supplies last in New York at the Madison Square Park and Upper West Side Shake Shack and Los Angeles locations in Glendale and El Segundo.