Preheat oven to 350°F.
In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Mix well.
Press the mixture firmly into a 9-inch pie pan, covering the bottom and sides evenly.
Bake crust for 8 minutes, then remove and let it cool.
In another bowl, whisk together sweetened condensed milk, sour cream, key lime juice, and lime zest until smooth.
Pour filling into the cooled crust and bake for 10 minutes, until tiny bubbles form on the surface.
Let the pie cool at room temperature, then refrigerate for at least 3 hours before serving.
Top with whipped cream before serving, if desired.