Preheat oven to 350°F (180°C). Lightly grease a 9×13-inch baking pan and line with parchment paper, leaving an overhang for easy removal.
Prepare the cookie layer. In a large mixing bowl, beat together softened butter, brown sugar, and granulated sugar until combined.
Add the egg, egg yolk, and vanilla extract. Mix until smooth.
Add flour and salt, mixing on low speed until combined. Fold in chocolate chips.
Press the cookie dough evenly into the bottom of the prepared baking pan.
Arrange Oreo cookies in a single layer over the cookie dough, covering as much surface area as possible.
For the brownie layer, place butter and chopped chocolate in a heatproof bowl. Microwave in 45-second intervals on medium power, stirring between each interval until smooth.
Allow the mixture to cool for about 5 minutes.
Whisk in granulated sugar and eggs until combined.
Add flour, cocoa powder, and salt, whisking just until incorporated.
Spoon brownie batter evenly over the Oreo layer and gently spread to cover.
Bake for 37–45 minutes, or until a toothpick inserted into the brownie layer comes out with a few moist crumbs.
Cool completely in the pan for at least 4 hours before slicing.
Lift brownies out using the parchment paper overhang and cut into squares.