Preheat your oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch baking pan with parchment paper, leaving some overhang on two sides to lift the bars out later. Lightly spray the parchment and sides with nonstick spray.

In a medium bowl, combine the graham cracker crumbs, granulated sugar, and fine sea salt. Stir with a fork until evenly mixed, then pour in the melted butter. Mix until all the crumbs look like damp sand and no dry spots remain.

Pour the crumb mixture into the prepared pan. Use your fingers or the bottom of a measuring cup to press the crumbs firmly and evenly across the bottom, pressing slightly up the sides if desired. Bake for 8–10 minutes, until fragrant and lightly golden. Set aside to cool for a few minutes while you make the filling.

In a large mixing bowl, whisk the egg yolks for about 1 minute until slightly thickened and a bit lighter in color. You can do this by hand or with an electric mixer on low speed.

Pour in the sweetened condensed milk and whisk until the mixture is smooth and fully combined, about 1–2 minutes, creating a custardy base.

Add the sour cream (or Greek yogurt), vanilla extract, and a pinch of salt. Whisk until the mixture is silky and no streaks remain. It should be pale yellow and very smooth.

Slowly whisk in the lime juice and lime zest. The mixture may look a bit thinner at first, but it will thicken slightly as it sits. Let the filling rest for about 5 minutes so the acid can react with the milk and start to set.

Give the filling a final stir, then pour it over the warm (not piping hot) graham cracker crust. Tilt the pan gently or use an offset spatula to spread it into an even layer. Tap the pan once or twice on the counter to release any large air bubbles.

Bake at 350°F (175°C) for 15–18 minutes. The edges should look slightly set and the center should still have a tiny jiggle when you nudge the pan, like firm gelatin rather than liquid. Do not overbake, or the bars may crack and become rubbery.

Remove the pan from the oven and cool at room temperature for about 30–45 minutes. Then cover and chill in the refrigerator for at least 3 hours, or overnight, until fully set.

Use the parchment overhang to lift the chilled slab out of the pan and place it on a cutting board. With a long, sharp knife, slice into 9 large bars or up to 16–20 smaller bars, wiping the blade between cuts for clean edges. Top with whipped cream, extra lime zest, thin lime slices, and/or toasted coconut flakes if desired.