Preheat your oven to 350°F (175°C). In a food processor, finely grind the graham crackers until you have crumbs. Combine the crumbs with melted unsalted butter and stir until well blended. Press this mixture into the bottom of a 9x9-inch square baking pan. Bake for 7-10 minutes, or until lightly golden.

Zest two key limes and juice them. In a mixing bowl, whisk together the sweetened condensed milk, vanilla extract, sour cream, and lime juice until smooth. Add in the egg yolks and whisk until combined.

Pour the filling over the cooled crust and spread it evenly. Bake for about 15 minutes, or until the edges appear set but the center jiggles.

Whip the heavy cream with sugar until stiff peaks form. Spread the whipped cream over the cooled Key Lime Pie Bars.

Slice into squares and serve chilled or at room temperature. Refrigerate for at least an hour before serving.