Preheat oven to 350°F (180°C).
In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Mix with a fork until evenly moistened. Press the mixture firmly into an 8×8-inch baking pan using the bottom of a measuring cup.
1 ½ cups graham cracker crumbs, 2 tablespoons white sugar, 5 tablespoons unsalted butter
Place the pan on a baking sheet and bake for 10 minutes. Remove from oven and let the crust cool slightly. Lower oven temp to 325°F (160°C).
In a clean bowl, whisk together egg yolks, sweetened condensed milk, key lime juice, zest, sour cream, and salt until smooth and creamy. Pour the filling over the slightly cooled crust and spread evenly.
3 large egg yolks, 1 ⅓ cups sweetened condensed milk, ½ cup key lime juice, 1 tablespoon key lime zest, 1 tablespoon full-fat sour cream, Pinch of salt
Bake for 20 minutes, or until the edges are set and the center has a gentle jiggle.
Let the bars cool completely on a wire rack (about 2 hours). Chill in the fridge if you like them extra cold!
Top with whipped cream and slice into squares before serving.
½ cup whipping cream