Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the sourdough discard, egg, key lime juice, and key lime zest until well combined.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Scoop dough into 1-inch balls and place them on the prepared baking sheet. Bake for 10-12 minutes, or until the edges are lightly golden.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
Keyword key lime cookies, lime cookies, sourdough key lime cookies