Preheat your oven to 350°F (175°C).
In a bowl, mix graham cracker crumbs, sugar, and melted butter until combined.
Press the mixture firmly into the bottom and sides of a 9-inch pie pan.
Bake the crust in the preheated oven for 10 minutes until lightly golden.
In another bowl, whisk the egg yolks until pale and fluffy.
Add sweetened condensed milk and mix until smooth.
Stir in the key lime juice and zest until well combined.
Pour the filling into the cooled crust and smooth the top.
Bake for an additional 15 minutes until set.
Allow to cool, then refrigerate for at least 3 hours before serving.