Whisk together flour, baking soda, and salt in a small bowl, and set it aside for a moment.
Mix together butter and brown sugar until light and creamy, then add an egg and vanilla extract.
Add the flour mixture, then stir in chocolate chips.
Spread the cookie dough in a square pan lined with parchment. Press Oreo cookies on top of the dough.
To make the brownie batter, combine chocolate and butter in a medium bowl, and heat until smooth and melted.
Whisk together eggs and two types of sugar until thick and frothy. Stir in the melted chocolate.
Add flour, salt, and cocoa powder to the chocolate mixture, and stir well.
Spread the brownie batter in the pan, then top with chopped cookies and chocolate chips. Bake and then cool completely.