Heat the oven to 350F, then grease an 8″ square pan. I used my Pyrex glass dish.
Combine the graham cracker crumbs, melted butter, sugar, and zest. Press into the prepared pan, pressing the crust mixture one inch up the side of the pan. Bake for 10 minutes. Allow to cool completely.
Once the crust is cooled combine the egg yolks and condensed milk until well mixed. Stir in the lime juice and combine. It will begin to thicken slightly.
Pour the filling into the crust, making sure the filling reaches the edges. Bake for 15 minutes, or until just set.
Cool to room temperature, then chill for at least one hour, overnight is better. Store in the fridge for up to 5 days.