• Preheat the oven to 325°F.
• Line an 8-inch square baking dish with parchment, leaving the extra paper over the sides to easily remove the bars from the pan.

Make the Crust

• In a small bowl mix the graham cracker, sugar and butter together well to combine.
• Pour the mixture into the prepared pan and press it gently to evenly cover the bottom of the pan. Using a heavy bottomed glass or measuring cup will help spread the crust evenly while pressing into the pan.
• Bake the crust for 10 to 12 minutes. Set aside to completely cool.
• Reduce the oven temperature to 300°F.

Make the Filling

• In a medium bowl, add the egg yolks and lime zest and mix at high speed with an electric mixer for about 4 minutes until very thick.
• Reduce to a medium speed and slowly add in the sweetened condensed milk. Mix for about 2 minutes.
• Reduce to low and add the key lime juice. Mix until just combined and then pour evenly over the top of the cooled graham cracker crust.

Bake the Bars

• Bake for 18 to 20 minutes until it is just set but slightly jiggly.
• Place on a cooling rack for about 2 hours. Note: Once the baked bars have cooled to room temperature, they can be refrigerated to cool completely.

Add the Whipped Topping

• Once cool, make the whipped cream topping in a medium bowl by mixing the heavy cream, sugar and vanilla with an electric mixer on a medium high speed until stiff peaks begin to form.
• Spread the topping over the cooled bars.
• Cover and refrigerate for at least 2 or 3 hours before serving.

Garnish and Serve

• Garnish with fresh lime zest when ready to slice, then slice into bars and serve.