In a mixing bowl, combine the graham cracker crumbs, granulated sugar, and melted butter. Mix until the crumbs are evenly coated.
Press the mixture into the bottom of an 8x8 inch baking pan to form a crust, ensuring it's firm and even. Bake for 8-10 minutes until lightly golden. Remove from the oven and set aside to cool slightly.
In another bowl, whisk together the sweetened condensed milk, key lime juice, lime zest, egg yolks, and a pinch of salt until smooth and fully combined.
Pour the key lime filling over the pre-baked crust, spreading it evenly.
Bake the bars for an additional 15-20 minutes or until the filling is set but still slightly jiggly in the center (it will continue to firm up as it cools).
Remove from the oven and let the bars cool to room temperature, then refrigerate for at least 3 hours or until fully chilled.
Once chilled, cut into squares and serve with a dollop of whipped cream and a slice of lime if desired.
Notes
Chill for at least 3 hours before serving for the best texture.