To take advantage of the offer, simply grab a single ShackBurger in the brand’s app or online with promo code “THANKYOU21,” and you’ll qualify for a second ShackBurger for free.

The brand’s signature ShackBurger features a pure beef patty topped with cheese, lettuce, tomato, and ShackSauce on a toasted potato bun.

Shake Shack’s BOGO ShackBurger promotion is valid at participating Shack locations nationwide through January 2, 2022.

Image via Shake Shack