1. Preheat oven to 350°F (175°C). Grease a 9-inch brownie pan and line with parchment paper.
2. For the cookie dough, cream butter, brown sugar, and granulated sugar with an electric mixer until fluffy.
3. Add egg and vanilla bean paste, mixing until smooth.
4. Fold in flour, baking soda, and salt. Stir in chocolate chips.
5. Spread cookie dough in pan, top with Oreos, and refrigerate.
6. Melt dark chocolate and butter together in a bowl over simmering water. Cool slightly.
7. Whisk eggs with granulated and brown sugar until pale and fluffy.
8. Slowly add melted chocolate to egg mixture, mixing on low speed.
9. Fold in cocoa powder, flour, and salt.
10. Pour brownie batter over Oreos and cookie dough.
11. Bake for 35-40 minutes until edges are set and the center is slightly gooey.
12. Cool completely before slicing.