Preheat the oven to 350°F (175°C). Line an 8x8-inch square baking pan with parchment paper leaving an overhang on two sides to lift the bars out later, or lightly grease the pan.

Combine graham cracker crumbs, granulated sugar, and salt in a medium bowl. Pour in the melted butter and stir until the mixture is evenly moistened and holds together when pressed.

Press the crumb mixture firmly and evenly into the bottom of the prepared 8x8 pan using the back of a measuring cup or a flat-bottomed glass to compact the crust (about 1/4 to 1/3 inch thick).

Bake the crust in the preheated oven for 8–10 minutes until set and just beginning to brown at the edges. Remove from oven and set aside while preparing the filling. Leave oven on.

Key Lime Filling

Lower the oven temperature to 325°F (160°C). In a medium bowl, beat the softened cream cheese with a whisk or a fork until smooth and free of lumps to ensure a silky filling.

Add the sweetened condensed milk to the cream cheese and whisk until fully combined and smooth.

Whisk in the egg yolks one at a time until incorporated, then add the key lime juice, lime zest, and vanilla extract. Whisk until the mixture is homogeneous and slightly thickened. Do not overbeat — just enough to combine.

Pour the filling over the hot baked crust, spreading it into an even layer with a spatula. Tap the pan gently on the counter to remove any air bubbles.

Bake at 325°F (160°C) for 15–18 minutes, or until the filling is set at the edges but still slightly jiggly in the center (it will continue to set as it cools). Avoid overbaking to retain a creamy texture.

Remove the pan from the oven and let cool on a wire rack for 30–45 minutes, then refrigerate for at least 2 hours (preferably 3–4 hours) until fully chilled and firm.

Finish and Serve

Using the parchment overhang, lift the chilled key lime slab from the pan and place on a cutting board. If you didn't use parchment, run a thin knife around the edges first.

Trim edges if desired, then cut into 16 equal squares for smaller portions (approximately 1.5-inch squares) or 9 larger squares. For clean slices, wipe the knife between cuts with a warm, damp towel.

If using, dollop or pipe about 1 tablespoon of lightly sweetened whipped cream onto each square and sprinkle a tiny pinch of extra lime zest over the top for garnish.

Serve chilled. Store leftovers covered in the refrigerator for up to 4 days.