Preheat oven to 350°F. Line a 9x13 inch pan with foil or parchment paper and spray with cooking spray.
Make the cookie dough: Stir melted butter with granulated and brown sugars until combined. Mix in egg, vanilla extract, baking soda, and salt. Stir in flour and then chocolate chips.
Press the cookie dough evenly into the bottom of the prepared pan. Arrange Oreos over the cookie dough layer evenly.
Make the brownie batter: Melt baking chocolate and butter in a microwave-safe bowl, stirring every 30 seconds until smooth. Stir in sugar. Add eggs, vanilla extract, and cocoa powder; mix well. Carefully stir in flour.
Spread the brownie batter over the Oreos evenly in the pan.
Bake for 30 to 40 minutes until the top looks set and the center is no longer jiggly. Let cool before slicing into bars.
Notes
This dessert is best served at room temperature for gooey layers. Store leftovers in an airtight container.