Preheat oven to 375°F. Zest one of the limes.
Beat the lime zest, 4 egg yolks, 2 cans sweetened condensed milk, and 1 cup of key lime juice until smooth and combined.
Pour into graham cracker crust. Bake for 20 - 23 minutes. Allow to cool completely. Refrigerate for 1 - 3 hours before serving.
To serve, top with whipped cream, lime slices, and lime zest