Preheat your oven to 350°F (175°C).
Line your baking sheet with parchment paper.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large mixing bowl, beat the butter until creamy. Add granulated and brown sugar, beating until light and fluffy.
Add the egg, vanilla extract, key lime juice, and key lime zest to the butter mixture. Mix until well combined.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.
Fold in the graham cracker crumbs with a spatula.
Scoop cookie dough onto the prepared baking sheet, leaving space between cookies. Bake for 10-12 minutes until lightly golden.
Remove from oven, let cool for a few minutes, then transfer to a cooling rack. Decorate with icing if desired.
