Preheat oven to 350 degrees F.

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. Drizzle the melted butter over the mixture and press evenly into a 9-inch pie plate. Bake for 10 minutes; cool to room temperature.

Once cooled, pour the lime filling into the crust. Bake until the center is set yet still jiggles slightly, 15 to 17 minutes. Cool to room temperature, then refrigerate for at least 3 hours.

Make the Whipped Cream: Whip the cream on medium speed until soft peaks form. Gradually add powdered sugar and vanilla until stiff peaks form. Pipe or spread on top of the pie, garnish with lime slices, if desired.