Preheat your oven to 350°F (175°C). In a mixing bowl, combine the graham cracker crumbs, granulated sugar, and melted unsalted butter until well blended. Press this mixture evenly into the bottom of a lined 8x8-inch baking pan to form a crust. Bake for 8-10 minutes, or until the edges are lightly golden and set.

In a separate bowl, whisk the egg yolks until they become lighter in color, about 1-2 minutes. Gradually incorporate the sweetened condensed milk and freshly squeezed Key lime juice, mixing gently to combine without overbeating. Add lime zest and a pinch of salt to enhance the flavor.

Once the crust has cooled slightly, pour the Key lime filling over it, spreading it evenly. Return the pan to the oven and bake for an additional 15-18 minutes, or until the filling is just set.

Remove the pan from the oven and let the bars cool at room temperature for about 30 minutes. Then, cover the pan with plastic wrap and refrigerate for at least 1 hour, preferably 2-3 hours or even overnight.

Once chilled, gently lift the Key Lime Pie Bars from the pan using the parchment paper. Slice into squares for serving, topping with a dollop of whipped cream made from heavy whipping cream, powdered sugar, and vanilla extract, and finish with additional lime zest.