Preheat your oven to 350°F (175°C). Prepare a 9x9-inch baking pan by lightly greasing it or lining it with parchment paper.

In a food processor, pulse the graham crackers until resembling fine crumbs. Melt the butter and combine with crumbs in a mixing bowl. Press into the bottom of the prepared pan. Bake for about 10 minutes, until golden.

While the crust bakes, juice fresh key limes and zest two of them. In a medium bowl, whisk key lime juice, sweetened condensed milk, vanilla extract, sour cream (or yogurt), and egg yolks until smooth.

Once the crust is done, pour the filling over it and bake for about 15 minutes, until edges are set but center jiggles slightly.

Allow to cool at room temperature for about 30 minutes, then cover and refrigerate for at least 2 hours.

Prepare the whipped cream topping by whipping chilled heavy cream and sugar with a hand mixer until stiff peaks form, about 3-5 minutes.

Once bars are chilled, lift them from the pan and spread whipped cream over the top. Slice into squares and serve chilled.