Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving overhang on sides.
In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Press firmly into the bottom of the prepared pan to form an even crust.
Bake crust for 10 minutes, then remove from oven and let cool slightly while preparing filling.
In a large bowl, whisk together sweetened condensed milk, key lime juice, lime zest, and egg yolk until smooth and well combined.
Pour filling over warm crust and spread evenly. Return to oven and bake for 20–25 minutes, or until set around the edges but still slightly jiggly in the center.
Remove from oven and cool completely at room temperature, then refrigerate for at least 2 hours before cutting into bars.
Optional: Top with whipped cream or meringue before serving.
Notes
For best flavor, use freshly squeezed key lime juice and zest. Store leftovers covered in the refrigerator for up to 5 days.