Pre-heat the oven to 170°C (325°F) and grease (or line) a square 8 x 8 inch baking tin.
Slowly heat the chocolate and butter in a pan until just melted, then add the coffee and milk. Leave the mixture to cool for about 10 minutes.
In a large bowl, whisk the sugar with the eggs, then quickly beat in the cooled chocolate mixture.
Sift the vanilla sugar, flour and cocoa together and stir into the wet ingredients, then mix until well combined. Lastly, throw in the chopped Mars Bar and give it one final stir to incorporate it.
Pour into the prepared tin and bake for around 20 minutes (until an inserted skewer comes out clean), but don't overcook! Remove from the oven and allow to cool completely (I usually leave my brownies in the fridge overnight as it makes for a cleaner cut).
For the frosting: beat the butter and cream cheese together until nice and light. Sift 100g (1 cup) icing sugar with the cocoa and beat into the other ingredients, gradually adding more icing sugar as needed.
When ready to assemble, using a round cookie cutter, cut out six circles from the brownie (the rest can be frozen and used to make trifles - link above). Spread around 1-2 tablespoons over each circle and then pop into the freezer for around ten minutes to set a little (tinned caramel never goes completely hard, but it will help the frosting stick better).
Top each brownie with a large dollop of frosting (or you could pipe it if you preferred). Mix the additional 2 tablespoons of caramel with a touch of milk, just until you have a somewhat runnier consistency, and then drizzle over the cakes. To finish, sprinkle with some chopped Mars Bars. Enjoy!