Pre-heat oven to 170 C/ 350 F. Butter and line an 8 inch square baking tin. Cut 4 Oreo cookies in half and keep aside. Let the rest be intact.
In a large microwave proof bowl, add butter and chocolate chips. Microwave at intervals of 20 seconds for up to a minute till the chocolate is melted and smooth. It's important to keep checking every 20 seconds and mixing with a spatula to ensure the chocolate does not burn.
Once the chocolate has melted, let the mixture cool for 5-10 minutes. Once it's cool to touch, add sugar, vanilla extract and whisk in the eggs one by one till everything is combined.
Add flour and cocoa powder and mix till just combined.
Now pour half the brownie mixture into the pan and line with cookies. You will need the cookies you cut in half to cover up the gaps.
Once the tin is lined with cookies, pour the remaining brownie mixture into the pan and even it out with the back of a spoon or spatula.
Bake for 40-45 minutes till a toothpick inserted into the middle comes out clean. Let the brownies cool completely before slicing them.