• Preheat the oven to 180°C/350ºF/Gas mark 4

For the brownie layer

• Grease & line a 20cm/8" square cake tin with parchment paper
• Melt the dark chocolate and butter together in a heatproof bowl over a saucepan of just simmering water (making sure the bottom of the bowl doesn't come into contact with the water)
• Stir to incorporate until the chocolate is completely melted and free of any lumps
• In another, larger mixing bowl, whisk the eggs and brown sugar together well
• Mix in the chocolate mixture
• Sieve together the flour and cocoa powder and fold into the mixture
• Pour the batter into the lined baking tin
• Cook for 15-20 minutes or until just baked
• Remove from the oven and set aside to cool, still in its tin

For the mint layer

• Melt the white chocolate and double cream together in a heatproof bowl over a saucepan of just simmering water (making sure the bottom of the bowl doesn't come into contact with the water). Stir to incorporate until the chocolate is completely melted and free of any lumps
• Stir in the mint extract
• Pour the mixture on to the brownie and spread evenly over the top with the help of a small offset palette knife
• Put the tin into the fridge and allow the mint layer to firm up

For the ganache layer

• Melt the dark chocolate and double cream together in a heatproof bowl over a saucepan of just simmering water (making sure the bottom of the bowl doesn't come into contact with the water). Stir to incorporate until the chocolate is completely melted and free of any lumps
• Once the mint layer has set, pour the ganache over and spread evenly with a small offset palette knife