Preheat the oven to 350ºF.

Grease a 9" pie pan with 1 Tablespoon butter.

In a food processor or blender, pulse together the crushed graham crackers, melted butter and sugar until combined.

Press the mixture into the greased pie pan.

Bake the crust for 8 minutes until golden brown.

Set crust aside and leave the oven on.

Use an electric mixer to whisk together the egg yolks and lime zest until fluffy, about 5 minutes.

Gradually whisk in the condensed milk until well incorporated.

Decrease the mixer speed and add the key lime juice and vanilla extract just until combined.

Pour the mixture into the graham cracked crust and bake for 10 minutes or until set.

Remove and place immediately in the freezer for 15 to 20 minutes before serving.