Preheat your oven to 160℃ (140℃ for fan ovens) then line an 8x8-inch tin with baking paper. Set aside for now.
Melt the dark chocolate and the butter together over low heat on the stove then set it aside to cool for around 10 minutes.
To a large bowl, add the sugar, eggs and vanilla extract. Use an electric whisk to whisk it all together until pale and silky.
Fold the melted chocolate mixture into the egg and sugar mixture. Then add the flour and the cocoa powder and mix gently until well combined.
Fold the Crunchie pieces and chocolate chips into the brownie batter. Then spread it out evenly in your tin.
Bake for around 35-40 minutes until the brownies have risen and are pulling away from the tin slightly. There should be a very slight wobble in the middle.
Leave them in the tin until cool then transfer them to the fridge overnight (or at least 6 hours) until solid.
Once set, cut them into squares and enjoy!