Preheat oven to 350 degrees. Place key lime juice in blender. Add egg yolks and softened cream cheese. Blend VERY well. Add sweetened condensed milk and blend again until very well mixed. Pour into graham cracker crust. Bake for 15 minutes at 350 degrees. Remove from oven and allow to cool for 15 minutes...then refrigerate until ready to serve. Right before serving top with whipped cream and lime zest if desired.