Step 1: Preheat your oven to 350°F (175°C). In a medium bowl, combine the graham cracker crumbs, granulated sugar, and melted butter. Mix until the crumbs are evenly moistened.
Step 2: Press the crumb mixture firmly and evenly into the bottom and up the sides of a 9-inch pie plate. You can use the bottom of a glass or your fingers to compact it. Bake the crust for 8-10 minutes, or until lightly golden. Remove from the oven and let it cool completely on a wire rack.
Step 3: While the crust cools, prepare the filling. In a large bowl, whisk together the egg yolks until they are light yellow and slightly thickened.
Step 4: Gradually whisk in the sweetened condensed milk until well combined and smooth.
Step 5: Slowly add the fresh key lime juice and key lime zest (if using) to the mixture, whisking continuously. The mixture will thicken as the lime juice reacts with the condensed milk and egg yolks. Continue whisking until the filling is smooth and uniform.
Step 6: Pour the key lime filling into the cooled graham cracker crust. Return the pie to the oven and bake for 10-15 minutes, or until the edges are set but the center still has a slight jiggle. Do not overbake, as it will continue to set as it cools.
Step 7: Remove the pie from the oven and let it cool completely on a wire rack to room temperature. Once cooled, cover the pie loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow it to set fully.
Step 8: Just before serving, prepare the whipped cream if desired. In a chilled bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract with an electric mixer on high speed until stiff peaks form. Top each slice of key lime pie with a dollop of whipped cream and a sprinkle of fresh key lime zest or a thin lime slice.