Preheat oven to 180 degrees C. Grease and flour a 23x33cm Swiss roll tray. In a small saucepan over medium heat, melt 110g of chocolate with 250g butter.
In a medium bowl, beat eggs and sugar with the melted chocolate mixture until well blended. Stir in the flour. Spread the batter evenly into the prepared tray. Bake for 17 to 20 minutes in the preheated oven. Cool in tray.
Beat the icing sugar with 90g of butter in a medium bowl. Gradually beat in milk, green food colouring and peppermint essence. Spread evenly over the brownie layer and refrigerate for at least 1 hour.
Melt 80g chocolate with 50g butter over medium heat or in a microwave oven, stirring frequently until smooth. Spread over the mint layer when completely cool. Chill again to set before cutting into bars.