Heat oven to 375 degrees F. Brush the graham cracker pie crust with the lightly beaten egg white, then bake for 5 minutes. Remove from the oven and set aside.
Reduce oven to 350 degrees F.
Meanwhile combine the egg yolks, key lime juice, lime zest, sweetened condensed milk, and sour cream in a medium bowl. Whisk until smooth. Pour into the prepared crust.
Bake for 25 to 27 minutes, until the top is mostly set, but the whole pie is still a little jiggly, then remove from oven and let cool to room temp.
Chill in fridge for at least three hours, then serve with whipped cream if desired. Enjoy!