Preheat your oven to 177 °C (175 °C). In a medium bowl, combine the graham cracker crumbs, melted butter, brown sugar, and a pinch of salt until well moistened. Press the mixture firmly into the bottom of an 8-inch cast-iron skillet or oven-safe baking dish. Bake for 7-8 minutes until set and fragrant, then set aside.

Heat the heavy cream in a small saucepan over medium heat just until warm and shimmering, but not boiling. Remove from heat, add the chopped dark chocolate, and let sit undisturbed for 2 minutes. Stir until completely smooth and glossy.

Pour the chocolate ganache over the baked graham cracker crust. Spread evenly using a spatula. Drizzle the warmed creamy peanut butter across the chocolate layer and gently swirl with a butter knife.

Dollop the marshmallow fluff generously over the top, spreading to cover most of the chocolate layer. Turn the oven to broil on HIGH. Place the skillet under the broiler for 1 to 2 minutes, watching constantly, until the marshmallow layer turns golden brown and toasted.

Remove from the oven carefully and let cool for 5 minutes before serving warm with whole graham crackers, sliced bananas, or pretzel twists for dipping.