For the crust:
- 1/4 cup sugar
- 1 1/2 cup graham cracker crumbs
- zest of one key lime
- 6 tablespoons salted butter, melted
For the filling:
- 1-14 oz. can fat free sweetened condensed milk
- 1/2 cup key lime juice
- 2 large egg yolks
Heat the oven to 350F, then grease an 8″ square pan, line it with parchment paper that hangs over the edge of the pan by a few inches (so you can remove the bars from the pan more easily).
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. Pull the bars out of the pan using the parchment paper and transfer to a cutting board. Cut into 16 squares with a serrated knife. To make cutting cleaner wipe the knife with a damp cloth between cuts.
Store in the refrigerator in an air tight container. They last for about five days.