Preheat your oven to 160℃ (140℃ for fan ovens) then line an 8x8-inch tin with baking paper.
Melt the dark chocolate and the butter together over low heat then set aside to cool for 10-15 minutes.
In a large bowl, whisk together the sugar, eggs and vanilla extract using an electric whisk. Keep whisking for a couple of minutes until the mixture becomes pale and silky.
Stir 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 in the Smarties and chocolate chips, saving a few for the top.
Spoon the batter into your lined tin and spread it out evenly. Top with the remaining Smarties and chocolate chips.
Bake the brownies for around 35-40 minutes until the edges are risen and pulling away from the tin slightly. Let them cool in the tin then transfer them to the fridge to set, ideally overnight but for at least 6 hours.
Once set, cut them into slices and enjoy!