Preheat oven to 350°F (180°C)
Mix graham cracker crumbs, ¼ cup sugar, and melted butter until dough forms
Press mixture into a 9-inch pie dish, forming an even crust
Bake crust for 8-10 minutes until golden, let cool
In a bowl, combine sweetened condensed milk, 4 egg yolks, ½ cup lime juice, 1 teaspoon zest, and ⅛ teaspoon salt
Whisk mixture briefly then cook over medium heat for 15-20 minutes, whisking constantly until custard thickens
Pour custard into cooled crust
Chill pie for 4 hours or overnight
Beat 1 cup chilled heavy cream until stiff peaks form, top pie with whipped cream and additional lime zest