Preheat your oven to 350°F (175°C). Line a 9x9 inch baking pan with parchment paper, leaving an overhang on two sides to easily lift the bars out later.

In a large mixing bowl, combine the graham cracker crumbs and granulated sugar. Pour in the melted unsalted butter and mix with a fork or spatula until the crumbs are evenly moistened and resemble wet sand.

Press the crumb mixture evenly into the bottom of the prepared baking pan. Use the bottom of a glass or your fingers to compact it firmly. Bake the crust for 10 minutes, then remove from the oven and set aside to cool slightly while you prepare the filling.

While the crust is baking or cooling, in a medium mixing bowl, whisk together the egg yolks and lime zest until light in color and fragrant.

Pour in the sweetened condensed milk and whisk until fully combined and smooth.

Gradually add the freshly squeezed Key lime juice, whisking continuously until the mixture thickens slightly. The acidity of the lime juice will react with the condensed milk, causing it to thicken.

Pour the Key lime filling evenly over the warm, baked graham cracker crust.

Return the pan to the oven and bake for an additional 15-18 minutes, or until the filling is set around the edges but still slightly jiggly in the center.

Remove the pan from the oven and let it cool completely on a wire rack at room temperature. This will take about 1-2 hours.

Once cooled, transfer the pan to the refrigerator and chill for at least 2 hours, or preferably 4 hours, to allow the bars to firm up completely. Chilling overnight is even better for the best texture.

Once fully chilled, use the parchment paper overhangs to carefully lift the bars out of the pan. Place them on a cutting board and slice into 16 squares (4x4).

Serve chilled, optionally garnished with a dollop of whipped cream and a thin lime slice or extra lime zest.