Preheat the oven to 350°F (177°C). Line an 8-inch square baking pan with foil or parchment paper, making sure to leave some overhang for easy removal. Spray the foil/parchment paper well with nonstick cooking spray and set aside.

Add the sliced butter and chopped chocolate to a large microwave-safe mixing bowl. Microwave in 30 second increments, stirring well after each one, until completely melted and smooth.

Stir in the sugar until well combined, then stir in the eggs and vanilla extract.

Sift in the flour, cocoa powder, and salt and continue mixing until just combined, stopping to scrape down the sides of the bowl as needed.

Pour half of the batter into the prepared baking pan and spread it around into one even layer.

Gently place 16 Oreos on top of the brownie batter. If needed, you can break the edges of some off to help them fit better.

Pour the remaining half of the batter on top and smooth it out. Break or chop the 6 remaining Oreos into smaller pieces and gently press them into the top.

Bake for 30 to 35 minutes or until a toothpick inserted into the center comes out mostly clean with just a few moist crumbs.

Remove from the oven and allow to cool completely in the pan on a wire cooling rack.