Prepare the Crust: Preheat your oven to 350°F (175°C). Mix the graham cracker crumbs, sugar, and melted butter in a bowl until the texture resembles wet sand. Press the mixture firmly into the bottom and up the sides of a 9-inch plate. Bake for 8-10 minutes until lightly golden, then let it cool slightly.
Make the Filling: In a large bowl, whisk the egg yolks and lime zest until thickened (about 2 minutes). Gradually whisk in the sweetened condensed milk, followed by the fresh lime juice. The mixture will thicken slightly as the acid reacts with the milk.
Bake: Pour the filling into the warm crust. Bake for 15-18 minutes. The pie is done when the edges are set but the center still jiggles slightly like gelatin when shaken.
Chill: Allow the pie to cool completely on a wire rack, then refrigerate for at least 3-4 hours (preferably overnight) to fully set the custard.
Serve: Just before serving, whip the heavy cream, powdered sugar, and vanilla until stiff peaks form. Pipe or spread the whipped cream over the chilled pie and garnish with lime slices or zest.