Preheat oven to 390F/180C. Line a 8″ square baking tin with greaseproof paper.
In a small bowl, break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn't burn. Alternatively, set up a double boiler. Do this by placing a heatproof bowl over a small pot of simmering water. Add the chocolate to the heatproof bowl and stir until melted.
In a large mixing bowl, whisk together melted butter and both sugars. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs, one at a time and mixing in between. Then add vanilla.
Check that the chocolate has cooled a little before adding in and combining. Add in the flour and cocoa powder and fold together.
Finally, stir in the milk chocolate chips. Pour the batter into the lined tin. Bake 22-25 minutes.
Remove from the oven, and cover the top of the brownies with the marshmallows and roughly crushed graham crackers/digestives. Bake a further 5 minutes until the marshmallows are golden and gooey.
Let cool before using a hot knife to slice.