Preheat your oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing the paper to overhang on two opposite sides for easy removal.

In a large mixing bowl, beat together the softened butter, peanut butter, and brown sugar until light and fluffy, about 2 minutes.

Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.

Reduce mixer speed to low and gradually beat in the flour until just combined. Do not overmix.

Transfer the dough to the prepared pan. Using lightly greased hands or a piece of wax paper, press the dough evenly into the bottom of the pan.

Arrange the Oreo cookies on top of the peanut butter dough in 6 rows of 4 cookies, pressing them in gently.

In a large microwave-safe bowl, combine the butter and chopped chocolate. Microwave on medium power in 45-second intervals, stirring between each, until completely melted and smooth.

Let the mixture cool for 2-3 minutes, then whisk in the granulated sugar until well combined.

Whisk in the eggs one at a time, mixing thoroughly after each addition.

Gently fold in the cocoa powder, flour, and salt using a rubber spatula, mixing just until no streaks remain.

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

Bake in the preheated oven for 50-55 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Check at 45 minutes to avoid overbaking.

Remove the pan from the oven and let the bars cool completely in the pan on a wire rack for at least 4 hours. This is crucial for clean slices.

Once cooled, lift the bars out using the parchment overhang. Transfer to a cutting board and slice into 18 rectangles.