Preheat the oven to 180°C (160°C fan). Grease and line a 20cm square baking tin with parchment paper.
In a heatproof bowl set over a pan of simmering water, melt the dark chocolate and unsalted butter together, stirring until smooth. Remove from heat and let it cool slightly.
In a separate bowl, whisk together the caster sugar, eggs, and vanilla extract until pale and fluffy.
Gradually pour the melted chocolate mixture into the egg mixture, stirring until well combined.
Sift in the plain flour and salt, then gently fold into the chocolate mixture until just combined.
Stir in the chopped peppermint chocolate until evenly distributed throughout the batter.
Pour the batter into the prepared baking tin and smooth the top with a spatula.
Bake in the preheated oven for 25-30 minutes or until the top is set and a skewer inserted into the centre comes out with a few moist crumbs.
Allow the brownies to cool completely in the tin before removing and transferring to a wire rack.
For the topping, melt the white chocolate in a heatproof bowl set over a pan of simmering water. Stir in the peppermint extract.
Once the brownies are cooled, drizzle the melted white chocolate over the top and sprinkle with crushed candy canes.
Allow the topping to set before slicing into squares and serving.
