1. Prepare

Preheat your oven to 350°F (180°C). Lightly grease a 9×13 inch (23×33 cm) baking pan, then line it with parchment paper leaving an overhang on the sides.

2. Make the Cookie Dough Layer

In a large bowl, beat together the softened butter, brown sugar, and white sugar until smooth. Add the whole egg, the egg yolk, and the vanilla extract, mixing until combined.
Gradually mix in the flour and salt on low speed just until combined. Turn off the mixer and stir in the chocolate chips. Spread the cookie dough evenly in the bottom of the prepared pan.

3. Add the Oreo Layer

Place the Oreo cookies in a single layer directly over the cookie dough. Press them down gently so they stay in place.

4. Make the Brownie Batter

Chop the dark chocolate finely. Place the chocolate and cubed butter in a heatproof bowl. Microwave in 45‎-second intervals on medium power, stirring between intervals until melted and smooth. Allow the mixture to cool about 5 minutes.
Whisk in the eggs and sugar. Then whisk in the flour, cocoa powder, and salt until just combined.

5. Assemble and Bake

Drop spoonfuls of the brownie batter over the Oreo layer and gently spread to cover as evenly as possible. Bake for 37‎–45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

6. Cool and Slice

Let the brownies cool completely in the pan (at least 4 hours) before lifting out with the parchment overhang and slicing.