Preheat oven to 350°F (175°C).
In a bowl, mix graham cracker crumbs, sugar, and melted butter until combined.
Press mixture firmly into a 9-inch pie dish. Bake crust for 10 minutes. Let cool slightly.
In a separate bowl, whisk together egg yolks and condensed milk until smooth.
Add key lime juice and zest. Whisk again until well combined.
Pour filling into baked crust. Bake for 15 minutes until set.
Cool to room temperature, then refrigerate for at least 4 hours or overnight.
Top with whipped cream before serving.