Mix the graham cracker crumbs with sugar (if using) and melted butter in a bowl until well combined and the mixture looks like wet sand.
Press the crumb mixture evenly into the bottom and up the sides of a pie dish, creating a firm, even crust. Use the back of a spoon or your fingers to press down.
Bake the crust in a preheated oven at 350°F (175°C) for about 8-10 minutes until fragrant and slightly golden. Remove from the oven and let it cool completely.
Whisk the egg yolks in a bowl until smooth and slightly thickened, about 1-2 minutes, until they look pale and creamy.
Add the sweetened condensed milk to the yolks and whisk until well combined, creating a smooth, thick mixture.
Stir in the key lime juice and lime zest, if using, until the mixture is uniform and slightly thickened.
Pour the lime filling into the cooled crust, smoothing the top with a spatula or the back of a spoon.
Bake the pie at 350°F (175°C) for 10-15 minutes, just until the filling is set but still slightly wobbly in the center.
Remove the pie from the oven and let it cool completely at room temperature, then refrigerate for at least 2 hours or overnight to allow the filling to firm up and flavors to deepen.
Slice the chilled pie with a sharp knife, revealing the crackly crust and creamy filling inside. Serve cold, garnished with lime slices or whipped cream if desired.
