Preheat oven to 350°F. Line the bottom and sides of an 8-inch square pan with parchment paper for easy removal (if not using a lining, grease the bottom and sides of the pan).

In a medium bowl, stir together the graham cracker crumbs and sugar. Add the melted butter and mix with a fork until the crumbs are well moistened. Press the mixture into the bottom of the prepared pan, using the flat bottom of a measuring cup or glass to evenly press the crust.

Bake for 7 minutes. Remove to a wire rack to cool while preparing the filling. Leave the oven on.

When crust is done pre-baking, whisk together the sweetened condensed milk, egg yolks, key lime juice, and zest. Pour into crust.

Before serving, make the whipped cream: Beat cold heavy whipping cream with powdered sugar on high speed until stiff peaks form. Pipe onto bars and garnish with lime slices before serving (optional).

Store bars in an airtight container in the refrigerator for up to 3 days. It’s best to top with whipped cream right before serving.

Can’t find key limes? Use bottled Nellie & Joe’s Famous Key West Lime Juice instead and the zest of regular Persian limes. You can also use regular lime juice as a substitute.