Quick service restaurant chain Shake Shack has considered its test marketing of its first chicken sandwich, the Chick'n Shack, a success, and will permanently add it to its menu.

The company in July introduced the Chick'n Shack sandwich at three locations in Brooklyn, New York. The sandwich will now be served at all Shake Shake locations. Most locations will debut the sandwich on January 14, while the remaining locations will start serving the sandwich shortly thereafter.

The Chick’n Shack includes a crispy chicken breast with lettuce, pickles and buttermilk herb mayonnaise. The chicken on the sandwich comes from birds never treated with antibiotics.

The inspiration behind the Chick’n Shack lies in Shake Shack’s heritage as a modern “roadside” burger stand. “The introduction of chicken marks a new era for Shake Shack, staying true to our core menu while continuing to innovate new items our guests have asked for,” said Shake Shack CEO Randy Garutti.

Garutti told CNBC that adding more chicken to the Shake Shack menu helped to diversify its offerings and give options to customers looking for a protein other than beef.

Shake Shack Culinary Director Mark Rosati added: “The Chick’n Shack embodies our modern approach to fine casual American cooking. It’s about providing a simple, pleasurable, uncomplicated experience, but with high-quality, responsibly sourced ingredients. It wasn’t until we found the chicken sandwich to be as flavorful as our ShackBurger that we knew we’d crafted something special that had earned its place on the menu. Since we introduced the Chick’n Shack in Brooklyn, guests have been asking us to bring it to all Shacks, and we couldn’t be more excited to finally do so.”