Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, letting it hang over the long sides to create handles. Lightly coat the parchment and any exposed sides of the pan with nonstick cooking spray.

In a medium bowl, stir together the graham cracker crumbs, granulated sugar, and fine sea salt. Pour in the melted butter and mix until the crumbs are evenly moistened and feel like wet sand. If the mixture seems dry, drizzle in up to 1 tablespoon more melted butter.

Pour the crumb mixture into the prepared pan. Use your hands or the bottom of a measuring cup to press the crumbs firmly into an even layer, going slightly up the sides if possible. Bake for 8–10 minutes, until fragrant and just a shade darker. Set aside to cool slightly while you prepare the filling.

In a large bowl, whisk the egg yolks for 30–45 seconds until slightly thickened and smooth. Add the sweetened condensed milk and whisk until the mixture looks uniform and glossy. Whisk in the sour cream (or Greek yogurt), vanilla, lime zest, and key lime juice until well combined and no streaks remain. The filling will be pourable; it will thicken as it bakes and cools.

Pour the creamy lime filling over the warm graham cracker crust. Use a spatula to spread it into an even layer, gently nudging it into the corners without banging the pan to avoid excess bubbles.

Return the pan to the oven and bake for 15–18 minutes, until the edges are set and tiny bubbles form around the sides. The center should still have a slight wobble when you gently jiggle the pan, like Jell-O rather than liquid. If large cracks appear, it is slightly overbaked but still fine to serve.

Place the pan on a wire rack and let the bars cool to room temperature, about 45–60 minutes. Cover the pan loosely (with foil or a lid) and refrigerate for at least 3 hours, or overnight, until fully chilled and set.

Once the bars are fully chilled, add the cold heavy cream, powdered sugar, and vanilla to a chilled mixing bowl. Using a hand mixer or stand mixer, whip on medium-high speed until soft to medium peaks form; the cream should be fluffy but still spreadable.

Use the parchment overhang to lift the chilled slab of bars out of the pan and onto a cutting board. Spread the whipped cream evenly over the top. Garnish with extra lime zest or thin lime slices. Cut into 16 large or 20–24 smaller bars, wiping the knife clean between cuts (a warm knife helps achieve neat slices). Serve chilled.