Preheat the oven to 180°C (160°C fan forced). Grease and line the base and sides of a 20cm square cake pan with baking paper.
Open the brownie box, remove enclosed frosting sachet if using Frosted Chocolate Brownie Mix and reserve for another use. Mix the brownie mix, butter and eggs together in a bowl until well blended. Spread the mixture evenly in the prepared cake pan and bake for 25-30 minutes or until a round bladed knife inserted into the centre comes out almost clean. Remove from the oven and leave the brownie to cool completely in the pan; about 1 hour.
To make the peppermint frosting, sift the icing sugar into a large bowl and add the 25g butter along with the peppermint essence and most of the milk. Add a few drops green food colouring or small amount of food colouring gel and beat together with a wooden spoon until smooth and creamy, adding the rest of the milk if needed. Beat in more food colouring if the mixture is not green enough. Use a small palette knife to spread the mixture over the cooled brownie square. Chill in the fridge for 15 minutes.
Make the chocolate topping. In a small microwave-proof bowl, microwave the chocolate chips, butter and golden syrup, uncovered, on high for 40-50 seconds, stirring halfway through until melted and smooth. Cool for 2-3 minutes, then spread the topping over the green frosting layer. Chill for at least 3 hours.
Use the baking paper to lift the brownie square from the pan. Place on a chopping board and cut into 16 squares, then cut each square diagonally to make 32 triangles to serve. Store any leftover triangles in an airtight container in a cool place for 2-3 days.