Step 1: Preheat your oven to 350°F (175°C). In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter. Mix until the crumbs are evenly moistened.
Step 2: Press the crumb mixture firmly into the bottom and up the sides of a 9-inch pie plate. Bake the crust for 8-10 minutes until lightly golden. Let it cool while you prepare the filling.
Step 3: In a large bowl, whisk together the sweetened condensed milk, egg yolks, and key lime juice until the mixture is smooth and slightly thickened.
Step 4: Pour the filling into the pre-baked and cooled crust. Spread it evenly with a spatula.
Step 5: Bake the pie for 15 minutes. The center should still have a slight jiggle but the edges should be set.
Step 6: Remove the pie from the oven and let it cool completely on a wire rack. Once cooled to room temperature, refrigerate for at least 4 hours, or preferably overnight, to set completely.
Step 7: Before serving, top with whipped cream and fresh lime zest if desired.