Preheat the oven to 350°F (175°C). Line an 8×8-inch baking dish with parchment paper, leaving some overhang on two sides for easy lifting. In a medium bowl, combine the graham cracker crumbs and granulated sugar. Pour in the melted butter and stir with a fork until the mixture looks like wet sand and all crumbs are evenly coated. Transfer the mixture to the prepared dish and press firmly into an even layer using the bottom of a measuring cup or a flat glass. Bake for 10 minutes, until the crust is lightly golden and fragrant. Remove from the oven and let cool for 10 minutes while you prepare the filling.
In a large bowl, whisk the egg yolks vigorously for about 1 minute until they’re thick and pale. Pour in the sweetened condensed milk and whisk until completely smooth and combined. Add the fresh Key lime juice and lime zest, then whisk gently but continuously for about 2 minutes. You’ll notice the mixture thickening slightly—that’s the acid from the lime juice reacting with the condensed milk, which is exactly what you want. The filling should be smooth and combined.
Pour the filling over the crust and bake until the center is just set. Let the bars cool for at least 4 hours in the fridge to fully firm up and develop that perfect sliceable texture. Top with a cloud of whipped cream and a sprinkle of lime zest, and you’ve got a dessert that tastes like a vacation.