Make the Filling: Whisk the lime zest and egg yolks together 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 that the mixture is evenly moistened. Press the crust mixture evenly into the bottom and up the 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. (At this point, the pie can be covered directly with plastic wrap sprayed with non-stick cooking spray and refrigerated for up to 1 day.)
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, 1 tablespoon at a time, 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 limes are more acidic than regular limes, which makes them tarter and much more aromatic. Key limes can be hard to find in some supermarkets, so if you can't locate them, use bottled key lime juice.