Preheat the oven to 350°F (175°C).

In a mixing bowl, combine graham cracker crumbs, melted butter, and granulated sugar. Press the mixture firmly into the bottom of a greased 9x9 inch baking dish to form the crust.

Bake the crust for about 10 minutes or until lightly golden.

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

Pour the key lime filling over the baked crust and spread evenly.

Bake for an additional 15-20 minutes, or until the filling is set but still slightly jiggly in the center.

Allow to cool at room temperature, then refrigerate for at least 2 hours before cutting into bars.

Dust with powdered sugar before serving, if desired.