Preheat your oven to 350°F (175°C). In a mixing bowl, crush graham crackers into fine crumbs and combine them with melted unsalted butter until mixture resembles wet sand. Press this mixture firmly into the base of a 9x9-inch square pan to form an even layer. Bake the crust for about 10 minutes until lightly golden and fragrant, then remove it from the oven and allow it to cool slightly.

While the crust cools, zest and juice key limes to capture their vibrant flavor. In a large bowl, whisk together the lime juice, sweetened condensed milk, vanilla extract, sour cream, and egg yolks until the mixture is silky smooth. Ensure there are no lumps.

Pour the lime filling over the warm crust, spreading it evenly with a spatula. Bake for 15-20 minutes until the edges are set and the center jiggles slightly. Let cool at room temperature for about 30 minutes before chilling in the refrigerator.

Whip chilled heavy cream with sugar until stiff peaks form to create a light, fluffy topping. Be careful not to overmix.

Once whipped cream is ready and bars have chilled for at least two hours, slice into squares. Top each square with whipped cream and garnish with extra lime zest if desired. Serve chilled.