Step 1: Preheat and Prep

Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, allowing extra to hang over the sides. This makes it easier to lift the bars out after baking.

Step 2: Make the Graham Cracker Crust

In a medium bowl, combine graham cracker crumbs, sugar, cinnamon (if using), and melted butter. Mix until the texture resembles wet sand. Press this mixture firmly into the bottom of your prepared pan, using the back of a spoon or the bottom of a measuring cup.

Step 3: Bake the Crust

Place the pan in the preheated oven and bake for 10 minutes. Remove and let it cool slightly while you prepare the filling.

Step 4: Whisk the Key Lime Filling

In a large bowl, whisk together the egg yolks until slightly thickened (1–2 minutes). Add the sweetened condensed milk and mix until fully incorporated. Then add the key lime juice, lime zest, and a pinch of salt. Stir until the filling is smooth and creamy.

Step 5: Pour and Bake

Pour the filling over the baked crust. Smooth the top with a spatula and bake for 15–18 minutes, or until the center is set but still jiggles slightly. The edges should be firm, and the top may have a few tiny bubbles.

Step 6: Cool and Chill

Allow the bars to cool to room temperature. Then transfer to the refrigerator and chill for at least 3 hours (overnight is best) to firm up.

Step 7: Slice and Serve

Using the parchment overhang, lift the bars out of the pan. Slice into squares using a sharp knife. Wipe the blade between cuts for clean slices. Garnish as desired with whipped cream, lime zest, or crushed graham crackers.