Mix the graham cracker crumbs with the melted butter in a bowl until well combined and the mixture resembles damp sand.
Press the crumb mixture evenly into a 9-inch pie pan, starting from the center and working outward to create a firm, even crust. Chill in the freezer for 15 minutes to set.
In a mixing bowl, whisk together the egg yolks and lime zest until the mixture is smooth and slightly pale, about 1-2 minutes.
Add the sweetened condensed milk to the egg yolk mixture, whisking until fully incorporated and silky.
Gradually pour in the fresh lime juice while whisking constantly; the mixture will thicken slightly and turn a vibrant greenish-yellow.
Pour the filling into the chilled crust, smoothing the top with a spatula. Tap the pie gently on the counter to remove air bubbles.
Bake the pie in a preheated oven at 350°F (175°C) for 15 minutes, or until the filling is just set but still slightly wobbly in the center.
Remove the pie from the oven and let it cool to room temperature, then refrigerate uncovered for at least 4 hours or overnight to allow the filling to fully set.
Once chilled, top with whipped cream if desired for a creamy, sweet finish. Slice and serve cold for a refreshing, tangy treat.
