Pre-heat the oven to 175℃ (350℉) and prepare a square/rectangle baking tin (mine was ) by lining it with grease-proof paper with a little overhang.
Slowly heat the chocolate and butter in a pan until just melted.
Combine the milk and instant coffee and pour into the melted chocolate mixture. Leave for a few minutes to cool slightly.
In a large bowl, whisk the white and brown sugar together with the eggs, then add in the cooled chocolate/butter mixture.
Mix the vanilla sugar, flour and cocoa and stir into the wet ingredients until well combined.
Pour into the tin and bake for around 24 minutes (see note 3). Take the brownies out of the oven and sprinkle with the marshmallows and return to the oven to bake for a further 4 minutes. Remove and allow to cool completely (do not try to remove the brownies from the pan when warm as the whole thing will collapse).
Drizzle with melted chocolate and then allow the chocolate to set. I left mine in the fridge overnight - if you do this, just take out of the fridge an hour before serving and cut into squares while it is still quite cold. Drizzle a little extra chocolate over for decoration (completely optional and certainly not necessary). Enjoy!
