Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving a little overhang.

In a mixing bowl, combine the graham cracker crumbs, sugar, and melted butter. Stir until the mixture resembles wet sand.

Firmly press this mixture into the bottom of the prepared pan to form an even layer. Bake for about 10 minutes, until lightly golden. Remove from the oven and set aside to cool slightly.

In another mixing bowl, whisk together the sweetened condensed milk, sour cream, Key lime juice, lime zest, and egg yolks until smooth and creamy.

Pour the filling mixture over the pre-baked crust, evenly spreading it out. Bake for an additional 15 minutes or until the filling is set but still slightly jiggles.

Remove from the oven and allow the bars to cool completely at room temperature. Afterwards, refrigerate for at least 2 hours or until chilled.

Once set, lift the bars out using the parchment paper, slice into squares, and serve!