Step 1: Prepare the Oven and Pan

Preheat your oven to 350°F (180°C). Lightly grease a 9x13 inch (23x33 cm) baking pan. Then, line it with parchment paper, leaving an overhang on the sides. Greasing the pan first helps the parchment paper adhere, preventing it from sliding around. This overhang will make it much easier to lift the brownies out of the pan later. Alternatively, you can simply grease the pan thoroughly if you don't have parchment paper.

Step 2: Make the Chocolate Chip Cookie Layer

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer. Beat until the mixture is light and fluffy, which usually takes about 2-3 minutes. This step is crucial for creating a tender cookie base. Don't skimp on the creaming process!

Next, beat in the whole egg, additional egg yolk, and vanilla extract. Using an extra egg yolk adds richness and chewiness to the cookie dough. Be sure to discard the white of the second egg – it's just the yolk we need for this recipe. Mix until well combined.

With the mixer on low speed, gradually beat in the all-purpose flour and salt. Mix until just combined; be careful not to overmix the dough, as this can result in tough cookies.

Turn off the mixer and stir in the chocolate chips using a rubber spatula or wooden spoon. Ensure the chocolate chips are evenly distributed throughout the dough.

Spoon the chocolate chip cookie dough into the prepared pan and spread it evenly with a spatula. It's okay if the dough is a little thick; just make sure it covers the entire bottom of the pan.

Step 3: Add the Oreo Layer

Arrange the Oreo cookies on top of the cookie dough, pressing them down slightly to ensure they adhere to the cookie layer. Aim for a neat arrangement. Typically you can get 7 rows of 4 cookies. Add a few half cookies on each row to completely cover the cookie dough layer. Don’t worry if you need to break a few to fill in the gaps. The goal is to create a solid Oreo barrier between the cookie and brownie layers.

Step 4: Prepare the Brownie Batter

Finely chop the dark chocolate (or semi-sweet chocolate). This will help it melt more evenly and prevent any large chunks in the brownie batter.

Place the chopped chocolate and cubed butter into a large heatproof bowl. Microwave on medium power for 45-second intervals, removing the bowl from the microwave between each interval and stirring the mixture. This gradual melting process prevents the chocolate from burning or becoming grainy. Do not microwave on high power!

Once the chocolate and butter are melted and smooth, cool the mixture for about 5 minutes. This is important because adding the eggs while the chocolate is still hot can cause them to cook.

Whisk in the eggs and granulated sugar into the slightly cooled chocolate mixture until well combined and glossy.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, and salt. This ensures that the dry ingredients are evenly distributed and prevents lumps in the brownie batter. If the flour and cocoa powder are lumpy, be sure to sift them before adding them to the wet ingredients.

Gradually whisk the dry ingredients into the wet ingredients until just combined. Be careful not to overmix; a few streaks of flour are okay. Overmixing can lead to tough brownies.

Step 5: Assemble and Bake the Brownies

Spoon the brownie batter over the Oreo layer, dropping spoonfuls across the pan. This prevents the Oreos from shifting around too much.

Gently spread the brownie batter evenly over the Oreos. This process makes it easier to spread the batter over the Oreos without disturbing them.

Bake the brownies in the preheated oven for 37-45 minutes, or until an inserted toothpick comes out with dry or with a few damp crumbs attached. The baking time will vary depending on your oven and how gooey you prefer your brownies. Keep a close eye on them during the last few minutes of baking to prevent them from overbaking.

Step 6: Cool, Slice, and Serve

Cool the brownies completely in the pan (at least 4 hours) before slicing. This is crucial for achieving clean slices and preventing the brownies from crumbling. Cooling them in the fridge can speed up the process.

Before slicing, lift the cooled brownies out of the pan using the overhang of the parchment paper. Place them on a cutting board.

Slice the brownies into 18 squares and serve.