Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.

Mix graham cracker crumbs, granulated sugar, and melted butter in a bowl until well combined. Press the mixture firmly into the bottom of the prepared pan.

Bake the crust for 10 minutes, then remove from oven and set aside.

In a separate bowl, whisk together the egg yolks, sweetened condensed milk, key lime juice, and lime zest until smooth.

Pour the filling over the baked crust and return to the oven. Bake for 15 minutes or until the filling is set.

Remove from oven and let cool to room temperature. Refrigerate for at least 3 hours to allow the bars to set.

Using the parchment paper overhang, lift the bars out of the pan and cut into squares before serving.