Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.
In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla, key lime juice, and key lime zest.
Combine the flour, baking powder, and salt; stir into the creamed mixture until just blended. Fold in the crushed graham crackers.
Drop by rounded teaspoonfuls onto the prepared baking sheets.
Bake for 12 minutes in the preheated oven, or until cookies are set and lightly browned at the edges.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to wire racks to cool completely.
Notes
These cookies are best served fresh but can be stored in an airtight container for up to 3 days. For an added touch, drizzle with a simple glaze made of powdered sugar and key lime juice.