Make the Filling: Whisk the lime zest and egg yolks in a medium bowl for 2 minutes. Whisk in the sweetened condensed milk, then the lime juice. Set aside at room temperature to thicken while you prepare the crust.
Make the Crust: In a medium bowl, stir together the graham cracker crumbs, brown sugar, and salt, ensuring no lumps of brown sugar remain. Drizzle the melted butter over the graham cracker mixture and toss to combine with a fork, ensuring the mixture is evenly moistened. Press the crust mixture evenly into the bottom and sides of a 9-inch pie plate, and pack tightly using the back of a measuring cup. Bake for 10 minutes; transfer to a wire rack to cool to room temperature.
Once the crust has cooled to room temperature, pour the lime filling into the crust. Bake until the center is set yet still wiggly when jiggled, 15 to 17 minutes. Return the pie to a wire rack; cool to room temperature. Refrigerate until well-chilled, at least 3 hours, or overnight.
Make the Whipped Cream: Using an electric mixer, whip the cream on medium speed until soft peaks form. At this point, add the powdered sugar, a small amount at a time, then the vanilla extract, while continuing to whip the cream until stiff peaks form. Decoratively pipe the whipped cream over the filling or spread the whipped cream evenly with a spatula. Garnish with lime slices, if desired, and serve. Cover leftovers with plastic wrap and refrigerate for up to 3 days.
Key Lime Juice: If you cannot get fresh key limes, use bottled key lime juice for this pie. The recommended brand is Nellie & Joe's (available at most grocery stores). While the flavor will be significantly different, you can also use regular limes or a combination of lime and lemon juice.
Key Lime Zest: If you cannot get fresh key limes, you can substitute regular lime zest.
Make-Ahead: This pie can be made up to 2 days in advance. Wait to top with whipped cream until the day you serve it.
How to Store the Pie: Refrigerate the pie, loosely covered with plastic wrap, for up to 3 days.
Freezing Instructions: Place the entire pie, uncovered, in the freezer until completely frozen. Then wrap well in plastic wrap and then a layer of foil (or place in a freezer-safe Ziploc bag) and store in the freezer for up to 1 month. Thaw in the refrigerator overnight.