1. Preheat Your Oven
Preheat your oven to 350°F. Then, line a 9x9-inch baking pan with parchment paper, leaving some overhang for easy removal.
2. Make the Crust
In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until well combined, and press the mixture firmly into the bottom of the prepared pan.
Bake for 10 minutes until lightly golden, then let it cool slightly.
3. Prepare the Filling
Using a stand mixer, whisk the egg yolks and key lime zest until the mixture has thickened. Then, add the key lime juice and sweetened condensed milk. Continue to whisk until the mixture is smooth and well combined.
4. Bake the Bars
Pour the filling over the baked crust, spreading it evenly. Bake for 15-20 minutes until the filling is set, but still wiggles as a whole when you shake the pan.
Let it cool completely at room temperature, then refrigerate for at least 3 hours to set.
5. Whip the Topping
In a large bowl, whip the heavy cream and powdered sugar until stiff peaks form. Spread the whipped cream over the chilled bars, and refrigerate for 20 minutes, or until ready to serve.
6. Serve
Lift the bars out of the pan using the parchment overhang. Cut into squares and enjoy! Garnish with extra lime zest if desired.