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. Leave to cool for 5-10 minutes until it comes to room temperature.
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 voluminous.
Fold the melted chocolate mixture into the egg and sugar mixture using a wooden spoon. Then mix in the flour and the cocoa powder.
Stir in the chocolate chips.
Spoon half of the batter into the tin and spread it out to the edges. Place the Bounty bars on top then cover them with the rest of the batter. You can cut off pieces of the bars if they're too large to fit all the way to the edges.
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!