Preheat the oven to 175°C and line a baking pan with parchment paper.

Melt the butter and dark chocolate for the brownie base in a saucepan over low heat, stirring until smooth.

Remove from heat and stir in the sugar, then add the eggs one at a time, mixing well after each addition.

Fold in the flour until just combined and pour the batter into the prepared baking pan.

Bake for 20-25 minutes, or until a toothpick comes out with a few moist crumbs.

Remove from the oven and immediately cover the top with marshmallows and broken pieces of graham crackers.

Broil for a few minutes until the marshmallows are golden brown, watching carefully to prevent burning.

For the ganache, heat the heavy cream until it just begins to simmer, then pour over the dark chocolate and let sit for a minute before stirring until smooth.

Drizzle the ganache over the toasted marshmallow layer and sprinkle with some crushed graham crackers.

Let cool before slicing and serving.