Line a square 8x8 baking pan with parchment paper to hang over all four sides. Then preheat the oven to 350 F/180 C.
Cream the butter, sugar, and light brown sugar for 2 minutes.
Mix in the egg yolk and vanilla extract until combined.
Fold in the flour, salt, baking soda, and chopped Oreos until just combined. Set aside.
Melt the butter and chocolate chips together until smooth and combined. Set aside.
Whisk the sugar, egg, and vanilla together until light and fluffy, about 2 minutes.
Stream the melted butter and chocolate into the bowl and mix until just combined.
Fold in the flour, cocoa, and salt until just combined.
Scoop two tablespoon sized portions of cookie dough into the prepared baking pan.
Scoop the brownie batter into the alternate spots in between the cookie dough.
Press down the top cookie dough balls so that they are the same height as the brownie batter.
Sprinkle extra chopped Oreos on top.
Then bake at 350 F/180 C for 25-27 minutes or until the cookie dough is golden brown and a toothpick inserted into the brownie batter comes out covered in a few moist crumbs. *(After 20 minutes, cover the top loosely with foil to keep the cookie from getting too brown.)*