Preheat oven to 350°F (175°C).

Finely grind graham cracker sheets to make 1 1/2 cups of crumbs.

Combine graham cracker crumbs, brown sugar, and 1/8 teaspoon of salt in a bowl.

Stir well to combine the dry ingredients.

Melt unsalted butter.

Drizzle the melted butter over the crumb mixture.

Toss with a fork until crumbs are moist.

Press the crumb mixture into the bottom and up the sides of a 9-inch glass or ceramic pie dish.

Bake for 8 minutes.

Let cool on a wire rack.

In a separate bowl, whisk together 1/3 cup of heavy cream, 2 teaspoons of Key lime zest, Key lime juice, sweetened condensed milk, egg yolks, egg, and the remaining 1/8 teaspoon of salt.

Whisk until the mixture is smooth.

Pour the Key lime mixture into the cooled crust.

Bake for 15 minutes.

Remove from oven and let cool for 15 minutes.

Refrigerate for 2 hours or until set.

In a clean bowl, combine powdered sugar, vanilla extract, and the remaining 2/3 cup of heavy cream.

Beat with an electric mixer at high speed until soft peaks form.

Dollop the pie with whipped cream.

Garnish with Key lime slices, if desired.