The year 2015 has been an exciting one for Shake Shack Inc (NYSE:SHAK). The much-loved burger chain IPO'ed in January, and the stock soared in its first months of trading. It hit an intraday high of $96.75 in May before sliding through the rest of the year to just $38, even with its 2015 low.

The business itself was thriving, as Shake Shack exceeded its goal of opening 10 new restaurants, easily beat earnings estimates, and its revenue is up 75% through the first three quarters of the year. Shake Shack's menu also featured some interesting additions. The most exciting of those seems to be the new ChickenShack sandwich, which debuted in July at Shake Shack's three Brooklyn locations.

The sandwich was Shake Shack's first-ever chicken dish, and was intended to be a limited-time offering at the time, but quickly sold out. Management spent two years working on the new menu item, which is made by slow-cooking the chicken with buttermilk, and then deep-frying to order. Lettuce, pickles, buttermilk herb mayo, and a Martin's potato roll complete the sandwich.

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The ChickenShack Source: Shake Shack.

Within two days, the ChickenShack had sold out, and was out of stock for a week as fans responded with rave reviews.  CEO Randy Garutti said the response to the ChickenShack has been "overwhelmingly positive" in the company's most-recent earnings call, and said that the sandwich had just been introduced at its four restaurants in Istanbul. 

There's no word yet on when the ChickenShack will expand nationwide; but when it does, the popular sandwich will likely provide a boost to Shake Shack's bottom line. The ChickenShack also demonstrates the company's ability to expand outside of burgers, paving the way for new menu items in other categories, and stretching the appeal of the brand.

A new kind of limited-time offering
Another promising idea the company showed off in 2015 was one-day partnerships with celebrity chefs to make one-of-a-kind burgers. In Atlanta, it teamed with local restaurateur Ford Fry to create the Carpetbagger burger, which was made up of two quarter-pound patties, comeback sauce, smoked onions, pickles, and two slices of American cheese. Finally, the burger was topped with a fried oyster. 

More recently, Shake Shack joined forces with two-star Michelin chef Sat Bains to serve up another unique creation in one of its London restaurants. Known as the Major Oak, the burger was inspired by the famous tree in the heart of Sherwood Forest. It featured a custom-blend Aberdeen Angus beef patty, topped with Beauvale bleu cheese, fresh shallots, a unique blend of ketchup, and smoked mayo. Only 500 burgers were made, and served on a first-come, first-served basis on October 9.

In the past, Shake Shack has partnered with other celebrity chefs including April Bloomfield, Daniel Boulud, and David Chang. Some more partnerships should come in the future as the chefs seem interested in teaming up with Shake Shack.

Looking ahead to next year, we could see more new menu items from Shake Shack as the company has also experimented with more mainstream burgers including the ShackMeister and the Roadside Shack burger, which have also won over fans. New menu items will continue to be an important weapon in Shake Shack's arsenal, as they will help keep the company's brand elevated above the rest of the fast-food fray.

Jeremy Bowman owns shares of Shake Shack. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.