Preheat your oven to 180ºC/350ºF. Line up square baking tin with baking paper, make sure it covers the sides as well.
Sift flour, cocoa and salt together in the bowl and set aside. Break chocolate to pieces and place into heat proof bowl with butter. Melt in the microwave in 30 second burst, always steering in between, until all the chocolate is fully melted.
Whisk eggs and both sugars with electric whisk or in standing mixer. It will take 3-4 minutes, until the eggs are fluffy, pale in colour and expanded. This will ensure you will have nice flaky crust after baking. Pour in chocolate, vanilla and whisk until fully incorporated. Add dry ingredients and fold in with spatula, that will prevent over-mixing.
Pour brownie mixture into your prepared baking tray and top it with broken Oreo biscuits. Bake for 20-25 mins. When you insert skewer into the brownie there should be some crumbs, the brownies will still cook whilst they cooling so it is better to underbake for gooey fudgy texture.
Make sure you let them cool completely in the pan before taking them out, otherwise they will break. You can chill before cutting with a sharp knife, to have nice clean edges.