Preheat the oven to 350°F (175°C). Line a 9×13-inch (23×33 cm) baking pan with parchment paper, ensuring an overhang on the sides for easy removal. Lightly grease the parchment with cooking spray or butter and set aside.

In a large microwave-safe bowl, combine the melted butter and 4 oz (113 g) of the chocolate chips. Microwave in 30-second increments, stirring after each, until the mixture is completely melted and smooth. Whisk in the cocoa powder until fully incorporated and set aside.

In a separate large mixing bowl, beat together the granulated sugar, brown sugar, eggs, egg yolk, and vanilla extract using an electric mixer on medium speed. Continue beating until the mixture becomes thick and pale, approximately 2 to 3 minutes.

Reduce the mixer speed to low and gradually add the melted chocolate-butter mixture to the egg and sugar mixture. Mix until well combined.

Add the flour and salt to the mixture. Gently fold in using a rubber spatula or wooden spoon until just combined. Stir in the remaining 4 oz (113 g) of chocolate chips. Do not overmix.

Pour half of the brownie batter into the prepared pan and spread it into an even layer. Roughly crush half of the Oreo cookies and sprinkle them evenly across the batter. Spoon the remaining batter over the crushed cookies and smooth the surface gently with a spatula. Top with the remaining crushed or whole Oreo cookies, pressing them slightly into the batter.

Transfer the pan to the preheated oven and bake for 35 to 40 minutes. The edges should appear set, and the center should be slightly soft. A toothpick inserted into the center should come out with moist crumbs attached, but not wet batter.

Allow the brownies to cool completely in the pan. Once cooled, use the parchment paper overhang to lift the brownies out. Slice into squares and serve as desired.