1Grease 8insx12ins rectangular slice pan. Line with parchment paper, extending paper above pan edges.
2Mix brownie mixture as directed on pack, using egg, water & vegetable oil. Smooth into prepared tin & cook in 350F oven for 20 mins, so still soft when taken from oven (or only cook for ¾ time stated on box if using different brand). Let cool completely.
Beat butter & sugar in a small bowl with electric mixer until light & fluffy. Meanwhile, sprinkle gelatine over water in a small heatproof jug. Stand jug in a saucepan of simmering water & stir until dissolved.
4With mixer running, gradually beat warm gelatine mixture into butter mixture until thick & creamy. Beat in peppermint essence to your taste & tint green with coloring. Spread evenly over cooled chocolate base.
Melt chocolate & oil & mix until smooth. Cool for 5 minutes. Spread over peppermint filling.