Preheat the oven to 180°C (160°C fan). Grease and line a 20 cm square baking tin with baking paper.

In a heatproof bowl, melt the butter and dark chocolate over a pan of simmering water. Stir until smooth, then set aside to cool slightly.

In a large mixing bowl, whisk together the caster sugar, light brown sugar, eggs, and vanilla extract until light and creamy.

Gradually fold in the melted chocolate mixture.

Gently fold in the plain flour, cocoa powder, and salt until just combined.

Pour the batter into the prepared tin and smooth the surface. Bake for 25 minutes or until a skewer inserted into the centre comes out with a few moist crumbs. Let the brownie cool completely in the tin.

In a mixing bowl, beat together the icing sugar, softened butter, milk, and peppermint extract until smooth and creamy. Add green food colouring, if using, to achieve the desired shade.

Spread the mint layer evenly over the cooled brownie base. Chill in the fridge for 30 minutes to firm up.

Melt the dark chocolate and butter together in a heatproof bowl over a pan of simmering water. Stir until smooth.

Pour the chocolate topping over the chilled mint layer and spread evenly.

Refrigerate for 1 hour or until the chocolate topping is set.

Once set, remove the brownie from the tin and cut into 12 squares. Serve and enjoy!