Prepare the Chocolate Chip Cookie Layer:
Preheat your oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large bowl, cream together the softened butter, brown sugar, and white sugar until light and fluffy. Add the egg, egg yolk, and vanilla extract, and mix until combined.
Gradually add the all-purpose flour and salt, mixing until fully incorporated. Stir in the chocolate chips.
Press the cookie dough evenly into the prepared baking pan, covering the bottom completely.

Add the Oreo Layer:
Place the 28 Oreo cookies on top of the cookie dough layer in a single, even layer. Gently press them into the dough.

Prepare the Brownie Layer:
In a medium saucepan, melt the cubed butter and dark chocolate over medium heat, stirring frequently. Once melted and smooth, remove from heat and whisk in the granulated sugar.
Add the eggs, one at a time, mixing well after each addition. Stir in the flour, cocoa powder, and salt until the batter is smooth and fully combined.

Assemble the Brownies:
Pour the brownie batter evenly over the Oreo layer, spreading it to cover the cookies.

Bake:
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter). Let the brownies cool completely in the pan on a wire rack before cutting into squares.

Serve:
Once cooled, lift the brownies out of the pan using the parchment paper overhang and cut into squares. Serve and enjoy!