Preheat oven to 350°F (175°C). In a bowl, mix graham cracker crumbs, sugar, salt, and melted butter until evenly combined. Press the mixture firmly into the bottom and sides of a 9-inch (23 cm) pie pan. Bake for 8 minutes, then let cool while preparing the filling.
In a large bowl, whisk the egg yolks until slightly thickened. Whisk in the sweetened condensed milk, key lime juice, lime zest, vanilla extract, and salt until smooth. Pour the filling into the cooled crust and smooth the top.
Bake at 350°F (175°C) for 12-15 minutes, until the center is just set (a slight jiggle is fine). Let the pie cool at room temperature for 30 minutes, then refrigerate for at least 3 hours (or overnight) until fully set.
Using a hand mixer or stand mixer, beat heavy whipping cream, powdered sugar, and vanilla on medium-high speed until stiff peaks form.
Spread or pipe the whipped cream on top of the chilled pie. Garnish with key lime zest and extra graham cracker crumbs if desired. Slice, serve, and enjoy!