Preheat your oven to 160℃ (140℃ for fan ovens) and line an 8x8-inch tin with baking paper.
Melt the dark chocolate and the butter together over low heat on the stove then set aside to cool for around 10 minutes.
To a large bowl, add the sugar, eggs and vanilla extract. Using an electric whisk, whisk it all together for a few minutes- until pale and increased in volume.
Fold the melted butter/chocolate mixture into the whisked mixture. Then add the flour and the cocoa powder and mix gently until well combined.
Break the Kinder chocolate and Kinder Bueno into segments then fold them into the brownie batter. Transfer to the batter to your tin and spread it out evenly.
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 for at least 6 hours, until solid. This will make them ultra fudgy and chewy.
Once set, remove them from the tin, cut into squares and enjoy!