Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease the parchment paper.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until fluffy and smooth, about 2–3 minutes. Beat in the eggs one at a time, then add the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Fold in the chocolate chips. Spread the cookie dough evenly over the bottom of your prepared pan.

Place a layer of Oreo cookies directly on top of the cookie dough, covering as much of the surface as possible.

In a medium saucepan, melt the butter and chopped chocolate over low heat, stirring constantly until smooth. Remove from heat and allow it to cool slightly.

In a large bowl, whisk together the granulated sugar, brown sugar, eggs, and vanilla extract. Slowly add the melted chocolate mixture, whisking continuously. Sift in the flour, cocoa powder, and salt, stirring until just combined.

Pour the brownie batter over the Oreo layer, spreading it evenly to cover all the cookies. Smooth the top with a spatula.

Bake in the preheated oven for 35–40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Allow the brownies to cool completely in the pan for about 1 hour. For clean slices, refrigerate the brownies for an additional 30 minutes before cutting.

Use the parchment paper overhang to lift the brownies from the pan, then slice into squares and enjoy.