Preheat your oven to 350°F / 280°C. Prepare a 13x9 baking dish by greasing it or spraying it and lining it with a parchment sling down the long side.

Crush 18 graham cracker sheets in a zipper lock bag (not sealed shut) with a meat mallet or rolling pin and dump into a mixing bowl. Add ⅔ cup white sugar, 2 tablespoons brown sugar, and a ¼ teaspoon ground cinnamon and whisk together. Pour ¾ cup melted butter over top of the mixture and whisk together again until a crumbly, sandy mixture forms. This can also all be done with a food processor if you prefer.

Pour the graham cracker mixture into the prepared 13x9 baking dish and spread around and press down into the dish until evenly covered. Use a dry measuring cup bottom to help flatten it out evenly if it helps. Place into the preheated oven and par bake the graham crust for 10 minutes and set aside.

While the crust is heating, prepare the key lime filling. In a large mixing bowl, add 2 large eggs and 4 large egg yolks and whisk well until no streaks of egg white remain. Add 2 14oz cans of sweetened condensed milk and whisk until fully combined. Now add 1 cup of key lime juice and 2 tablespoons of lime zest and continue whisking until it no longer looks separated, and everything is well combined.

When the crust comes out of the oven, you can add the key lime filling right away, no need to wait for it to cool first. Pour the key lime filling over the graham cracker crust and use a large spoon or spatula to make sure it covers the crust corner to corner and creates an even layer. Return to the oven and bake for another 15-20 minutes.

The key lime pie bars are cooked when the center just barely jiggles anymore. Remove from the oven and set the baking dish on a cooling rack to cool to room temperature, about an hour or so. Once cooled, cover and place in your refrigerator for at least 2-3 hours, or overnight.

When ready to serve, remove from fridge and lift the key lime bars out of the baking dish with the parchment sling. Use a sharp knife to cut into squares and top each bar with extra lime zest, thinly sliced limes, or fresh whipped cream if you prefer.