Preheat your oven to 350°F (175°C). In a medium bowl, mix together the graham cracker crumbs, melted butter, and granulated sugar until well combined. Press the mixture firmly into the bottom of a 9x9-inch baking dish to form an even crust.
Place the dish in the preheated oven and bake for 10-12 minutes, or until the crust is lightly golden. Remove from the oven and allow to cool slightly.
In a large mixing bowl, whisk together the sweetened condensed milk, key lime juice, egg yolks, lime zest, and a pinch of salt until smooth and creamy.
Pour the key lime filling over the cooled crust, spreading it evenly with a spatula.
Bake in the oven for an additional 15-18 minutes, or until the filling is set but still has a slight jiggle in the center.
Remove from the oven and allow to cool at room temperature for about 30 minutes. Then, refrigerate for at least 2 hours to fully set.
Once chilled, slice the bars into squares or rectangles.
If desired, top each bar with a dollop of whipped cream and a slice of key lime for a decorative touch.
Notes
For best results, use freshly squeezed key lime juice.