Preheat oven to 350 degrees. Line an 8" square pan with parchment, if desired.

Mix graham cracker crumbs, sugar and butter in a bowl.

Press mixture into the bottom of the pan.

Bake the crust for 10 minutes. Let cool completely before making filling.

Mix together egg yolks and key lime zest for about 5 minutes or until thick.

Mix in condensed milk until well combined, about 3 minutes.

Add key lime juice and mix until just combined.

Pour filling over cooled crust.

Bake for 10 minutes or until set.

Let cool and then refrigerate until ready to cut.

Cut into squares and top with whipped cream and a slice of key lime, if desired.