Heat the oven to 350°F (175°C). Combine the graham cracker crumbs, sugar, melted butter, and salt, then press the mixture firmly into the bottom and sides of a 9-inch pie plate.

Bake the crust for 8–10 minutes, then let it cool briefly. Whisk the egg yolks until smooth, add the condensed milk, Key lime juice, and zest, and mix just until combined.

Pour the filling into the warm or cooled crust. Bake for 15–18 minutes, until the edges are set and the center still has a slight wobble.

Cool the pie at room temperature, then refrigerate it for at least 3 hours. For the cleanest slices and firmest texture, chill it overnight.

For the optional topping, beat the cold heavy cream with confectioners' sugar and vanilla until soft or medium peaks form. Spoon or pipe it over the chilled pie and garnish with lime zest or thin lime slices.