Preheat your oven to 350°F (175°C). In a food processor, crush the graham crackers until fine crumbs form, then combine with melted butter in a mixing bowl. Press this mixture into the bottom of a greased 9x9 inch baking pan and bake for 8-10 minutes, until golden brown.

Zest and juice the key limes, measuring about 1/2 cup of juice. In a separate mixing bowl, whisk together sweetened condensed milk, vanilla extract, sour cream, and egg yolks until smooth.

Once the crust is lightly browned, pour the key lime filling over the hot crust. Spread it evenly. Return the pan to the oven and bake for 10-15 minutes until the edges are set but the center jiggles slightly.

Let the bars cool at room temperature for about 30 minutes, then refrigerate for at least 2 hours to fully set. Whip the heavy cream in a chilled bowl until stiff peaks form and spread over the cooled bars.

Remove the bars from the refrigerator, cut into squares or rectangles for serving, and garnish with fresh lime slices or mint leaves if desired.