Preheat your oven to 350°F (177°C). Line an 8x8 inch baking pan with parchment paper, leaving a little overhang on two sides to easily lift the bars later.
In a medium bowl, combine the crushed graham crackers, light brown sugar, and zest from one key lime. Pour in the melted butter and stir until everything is well mixed and looks like damp sand.
Press this graham cracker mixture firmly and evenly into the bottom of your prepared pan. Bake for just 10 minutes to help set the crust. Let it cool on a wire rack while you prepare the creamy filling.
In the bowl of a stand mixer (or using a hand mixer), beat the room temperature cream cheese until it's wonderfully smooth. Then, on low speed, gradually beat in the sweetened condensed milk, key lime juice, and zest from two key limes. Continue mixing slowly until the filling is perfectly smooth and lump-free.
Carefully pour the zesty key lime filling over your cooled graham cracker crust. Place the pan back in the oven and bake for another 10 minutes to help the filling firm up.
Allow the bars to cool completely at room temperature. Once cool, cover the pan and refrigerate for a minimum of 4-6 hours, but ideally overnight. This crucial step ensures your bars are perfectly set and easy to slice!
If you desire a luscious topping, whip the heavy cream with the tablespoon of sugar and two teaspoons of key lime juice until soft to medium peaks form.
Once thoroughly chilled, use the parchment paper overhang to lift the entire slab of key lime bars from the pan. Cut into 16 squares. Top each bar with a dollop of fresh key lime whipped cream and an extra sprinkle of key lime zest if you wish. Enjoy!