Generously grease a 28x18cm tray with butter and set aside. In a pan, melt the butter, sugar, cocoa and sugar.
Meanwhile, crush the disgestive biscuits, either in a sandwich bag with a rolling pin, or blitz in a blender. Add to the melted mixture and stir until combined.
Transfer the base mixture to your prepared tray and using a wooden spoon, press the mixture into the tray, ensuring you get right into the corners and have an even finish. Place the base into the fridge to chill for 1 hour.
Once the base has cooled, prepare the filling but sifting the icing sugar into a large bowl. Add 1 tsp of hot water at a time until you have a spreadable consistency; 3-4 tsp of water should be enough but add more or less as required.
Add the peppermint extract and green food colouring (optional) to the icing and mix until well combined. Spread the icing evenly over the biscuit base and return to the fridge for another 1 hour to set.
Once the filling has set, remove from the fridge and set aside. Over a bain marie (or in short bursts in the microwave) melt the dark chocolate before pouring and evenly spreading on top of the filling. If desired, top with some crushed Mint Aero.
Allow the peppermint slice to set in the fridge for at least 1 hour but ideally overnight. Once set, cut into your desired size.