• Gather all ingredients and line an 8-inch square pan with parchment paper; set aside.

Graham Cracker Crust:

• Preheat the oven to 350°F (175°C). Prepare the pan by lining it with parchment.
• Combine the graham cracker crumbs, sugar, cinnamon, nutmeg, and salt in a bowl. Stir in the egg white and melted butter until the mixture holds together.
• Press the crumb mixture evenly into the prepared pan to form a compact layer.
• Bake the crust about 10 minutes, until set and lightly golden. Remove from the oven and allow it to cool while you prepare the filling.

Key Lime Filling:

• Increase the oven temperature to 375°F (190°C).
• In a large bowl, beat the room-temperature cream cheese with the paddle or a mixer until smooth and creamy.
• Add the egg yolks and beat with the whisk or paddle attachment until fully incorporated and the mixture is smooth—about 1 minute—scraping the bowl as needed to eliminate lumps.
• Pour in the sweetened condensed milk, lime juice, and lime zest. Whisk thoroughly, scraping the sides and bottom, until the filling is thick, silky, and uniform.
• Pour the filling over the cooled crust and return to the oven. Bake 15–18 minutes, until the edges are set and the center still has a slight wobble. Check at 15 minutes and cover loosely with foil if it needs a minute or two more.
• Allow the bars to cool in the pan for about 1 hour, then transfer to the refrigerator to chill for 2–3 hours or overnight for best texture.
• Lift the bars from the pan using the parchment, slice into serving pieces, and top with whipped cream and extra lime zest if desired.
• Store leftovers in an airtight container in the refrigerator for up to 4 days.