1. Preheat the Oven: Preheat your oven to 350°F (180°C). Lightly grease a 9×13 inch (23×33 cm) pan and line it with parchment paper, leaving an overhang on the sides for easy removal.

2. Make the Chocolate Chip Cookie Layer:

• In a large bowl, beat together the softened butter, brown sugar, and white sugar until well combined and fluffy.
• Add the egg, egg yolk, and vanilla extract. Mix until just combined.
• Gradually mix in the flour and salt until a dough forms. Be careful not to overmix.
• Fold in the chocolate chips evenly throughout the dough.
• Spread the cookie dough evenly across the bottom of the prepared pan using a spatula.

3. Add the Oreo Layer:

• Arrange the Oreo cookies in a single layer on top of the cookie dough, pressing them gently into the dough. Ensure the Oreos cover the entire surface.

4. Make the Brownie Layer:

• Melt the butter and dark chocolate together, either in a microwave at 45-second intervals or using the double boiler method. Stir until smooth and let cool slightly.
• In a separate bowl, whisk together the eggs and granulated sugar until combined.
• Gradually add the melted chocolate mixture to the egg mixture, whisking continuously.
• Sift together the flour, cocoa powder, and salt. Fold these dry ingredients into the wet mixture until just combined.
• Drop spoonfuls of the brownie batter over the Oreo layer and spread evenly with a spatula.

5. Bake the Brownies:

• Bake in the preheated oven for 37-45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
• Allow the brownies to cool completely in the pan, at least 4 hours.

6. Slice and Serve:

• Once cooled, use the parchment paper overhang to lift the brownies out of the pan. Place on a cutting board and slice into squares or rectangles.
• Store any leftovers in an airtight container at room temperature for up to 4 days, or freeze for up to 3 months.