In a medium saucepan, combine sugar, cornstarch, and salt. Gradually whisk in milk until smooth.

Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and begins to boil.

In a small bowl, temper the egg yolks by slowly whisking in a few tablespoons of the hot milk mixture. Then, return the egg mixture to the saucepan and continue cooking for 2 minutes.

Remove from heat and stir in butter, key lime juice, and vanilla extract. Mix until smooth. Let the pudding cool slightly.

In a separate bowl, combine graham cracker crumbs, melted butter, and sugar (if using). Mix until evenly coated.

To assemble the parfaits, spoon a layer of graham cracker crumbs into the bottom of each serving glass.

Add a layer of key lime pudding over the crumbs. Repeat layers as desired, finishing with pudding on top.

Optional: Top each parfait with whipped cream, lime zest, and fresh mint leaves for garnish.

Chill in the refrigerator for at least 30 minutes before serving to allow flavors to meld and the pudding to set.