Step 1: Prep the Pan
Preheat the oven to 350°F (180°C). Grease a 9×13 inch baking pan, then line with parchment paper, leaving an overhang on the sides for easy removal. Lightly grease the parchment as well.

Chocolate Chip Cookie Layer

1. In a large mixing bowl, beat together the softened butter, brown sugar, and white sugar until light and fluffy.

2. Add the egg, egg yolk, and vanilla extract, and beat until combined.

3. Mix in the flour and salt on low speed until fully incorporated.

4. Stir in the chocolate chips with a rubber spatula or wooden spoon.

5. Spoon the dough into the prepared pan and spread into an even layer using a spatula.

Oreo Layer

1. Arrange the Oreo cookies on top of the cookie dough layer.

2. Press them gently into the dough. Aim for 7 rows of 4 cookies. You can break a few Oreos in half to fill in any gaps and cover the entire surface.

Brownie Layer

1. Finely chop the dark chocolate and place it in a heatproof bowl with the cubed butter.

2. Microwave on medium power in 45-second intervals, stirring between each until fully melted and smooth. Do not use high power.

3. Let the mixture cool for 5 minutes to avoid scrambling the eggs.

4. Whisk in the sugar, then the eggs, one at a time.

5. In a separate bowl, whisk together the flour, cocoa powder, and salt (sift first if lumpy).

6. Stir the dry mixture into the chocolate mixture until just combined.

7. Spoon the brownie batter evenly over the Oreo layer in dollops, then gently spread it out with a spatula. This helps distribute the batter without shifting the cookies.

Bake and Cool

1. Bake in the preheated oven for 37–45 minutes. Check with a toothpick—it should come out clean or with just a few moist crumbs.

2. If the middle wobbles when you shake the pan, it needs more time.

3. Allow brownies to cool fully for at least 4 hours in the pan before slicing.

4. Once cool, use the parchment overhang to lift out the slab. Slice on a cutting board into squares.