Add the butter and chocolate chips to a heatproof bowl. Place the bowl over a pot of warm water (make sure the water is hot but not boiling) and gently stir until the chocolate is completely melted and smooth.
Remove from heat and let the chocolate mixture cool for 5–10 minutes.
Add the granulated sugar and whisk until well combined.
Add the eggs, egg yolk, and vanilla extract. Whisk for 1–2 minutes until the mixture becomes smooth and glossy.
Sift in the cocoa powder, flour, and salt. Gently fold until the dry ingredients are just incorporated. Be careful not to overmix.
Fold in the chopped Oreo cookies, making sure they are evenly distributed throughout the batter.
Pour brownie batter into a parchment-lined baking pan and spread it into an even layer. Top with the reserved Oreo pieces.
Bake for 20–23 minutes, or until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs.
Let the brownies cool completely before slicing. Enjoy!