Preheat your oven to 180°C/160°C(fan)/350°F/Gas 4. Line a 25x20cm (10x8") baking tray with baking paper.
Place 150 g Chocolate and 200 g Butter into a microwave safe bowl. Heat in the microwave until fully melted. Alternatively, place the bowl over a saucepan of boiling water and melt.
In a separate bowl, add 275 g Light brown sugar and 3 Egg. Whisk together for 3-4 minutes, until light and fluffy.
Pour the chocolate mixture into the egg mixture with 1 teaspoon Vanilla extract. Fold together very gently.
Add 100 g Plain flour and 50 g Cocoa powder to the mixture.
Carefully fold in the flour and cocoa. Add 2 pack Kinder Bueno bars and 50 g Chocolate. Carefully mix again.
Pour the batter into the lined baking tray and top with 1 pack Kinder Bueno bars.
Bake for 25-30 minutes. Remove from the oven and allow to cool. The brownies will seem uncooked when removed from the oven, but they will continue to cook in the tray.
Notes
If you want to continue with the hazelnut theme, then you can put a drizzle of melted Nutella on top of the brownies before serving.
When beating the eggs and sugar, make sure to beat it until it doubles in size.
When adding the flour and cocoa, fold it in very gently. Do not over-mix, otherwise the brownies will be too dense.
Let the brownies cool before serving. I would recommend putting them in the fridge for a couple of hours (or even overnight) before cutting.
Make sure your eggs are room temperature before baking.
Make sure to line your tin with grease proof paper so the brownies come out easily.