Preheat the oven to 350°F (175°C). In a mixing bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until well combined.

Press the mixture firmly into the bottom of a greased 9x9 inch baking pan. Bake for 10 minutes and then remove from the oven.

In a bowl, whisk together the eggs, sweetened condensed milk, key lime juice, key lime zest, and sugar until smooth.

Pour the filling over the baked crust and return to the oven. Bake for an additional 15-20 minutes, or until set.

Allow the bars to cool at room temperature, then refrigerate for at least 2 hours before slicing.

Cut into squares and serve chilled, optionally garnished with whipped cream or lime slices.