Pre-heat the oven to 170°C (325°F) and prepare an 8x8? tin by lining it with grease-proof paper or by greasing.
Slowly heat the chocolate and butter in a pan until just melted, then add the coffee. Leave to cool for about 10 minutes.
In a large bowl, whisk the sugar with the eggs, then add in the cooled chocolate mixture.
Sift the vanilla sugar, flour and cocoa and stir into the wet ingredients until well combined.
Pour into the tin and bake for around 20-25 minutes (until an inserted skewer comes out clean). Remove from the oven and allow to cool completely. If you have the time, let the brownie sit in the fridge over night.
For the frosting: beat the butter and icing sugar together and add just enough milk to give the frosting a thick, spreadable consistency. Add the food colouring and peppermint extract and beat in. Lather over the brownie and then sprinkle with finely sliced mint chocolate. Cut into squares and then serve.