Preheat oven to 350 degrees..
Mix graham cracker crumbs, 1/3 cup sugar and butter in small bowl.
Pour mixture into 9 inch pie pan pressing firmly on bottom and up the sides of the pan.
Bake for 10 minutes. Allow to cool completely.
Preheat oven to 300 degrees
Blend sweetened condensed milk and egg yolks at slow speed until smooth.
Add Key Lime Juice slowly while blending thoroughly.
Blend in lime zest if using.
Pour into pie crust.
Bake in oven 15 minutes. Let pie cool for 20 minutes before refrigerating. Best if chilled 8-10 hours before serving.
Top with whipped cream and lime zest, if using