To a small bowl, add graham cracker crumbs, brown sugar, and melted butter. Mix, ensuring that every crumb is coated with butter. Press the crumbs into the bottom and up the sides of a 9-inch pie pan. Place the crust into the refrigerator to chill for at least 1 hour while you make the filling.

In a large mixing bowl, beat the cream cheese on medium speed until smooth and creamy, about 30 seconds to 2 minutes. Scrape the bowl.

Add the sweetened condensed milk, key lime juice, and optional key lime zest. Mix until fully combined, stopping to scrape down the sides of the bowl as needed.

In a separate mixing bowl, add the heavy whipping cream. Using a handheld mixer or stand mixer fitted with the whisk attachment, mix on low-medium speed for 30 seconds to 1 minute then increase the speed to medium-high and continue mixing until stiff peaks form.

Gently fold one-third of the whipped cream into the cream cheese mixture until just combined, then fold in the remaining whipped cream until fully combined.

Pour the filling over the graham cracker crust and spread it into an even layer. Cover with plastic wrap and transfer to the refrigerator to chill for at least 4-6 hours or overnight until set.

When ready to serve, garnish with key lime slices, fresh mint, and whipped cream.