Preheat the oven to 350°F (175°C).

In a large bowl, combine graham cracker crumbs, granulated sugar, and melted butter until the texture resembles wet sand. Press mixture into a lined 8x8-inch pan. Bake for 8 to 10 minutes until golden brown and let cool completely.

In a medium saucepan over medium heat, combine sweetened condensed milk, brown sugar, and unsalted butter. Stir continuously until thickened and smooth, about 5 to 7 minutes. Pour over the cooled crust and let set.

Heat heavy cream in a small saucepan until warm but not boiling. Pour over white chocolate chips in a bowl and let sit for a minute. Stir until smooth. Add cream cheese, key lime juice, and key lime zest, whisking until fully combined.

Pour the white chocolate lime mixture over the middle layer, spreading it evenly.

Cover with plastic wrap and refrigerate for at least 4 hours to set, preferably overnight.

Once set, lift from the pan using parchment paper, cut into squares, and garnish if desired.