Put the graham crackers into a food processor and pulse until fine. Pour the melted butter in and pulse a few times until it’s all combined. Tip the cracker mix into a large oven dish and use a glass to spread it over the bottom and slightly up the sides. It doesn’t need to come right to the top. Put the dish into the freezer for 30 minutes.
Preheat the oven to 180C (160C fan). Put the caster sugar, butter and chocolate into a large saucepan and heat gently, stirring often, until it’s all melted and combined. Take off the heat and beat in the eggs and double cream, then finally the flour.
Put the oven dish onto a flat baking tray and pour the chocolate filling in. Bake for 10 minutes, until it has just set on top, then pile on lots of marshmallows and return to the oven for another 10 minutes, until the marshmallows are bubbling and browned. Carefully take the baking tray out of the oven - some of the marshmallows may have dribbled over the sides.
Serve hot, spooned into bowls, with Graham crackers to dip in and scoop up the gooey filling.