Preheat the oven to 350°F.
Coat an 8x8 baking pan with cooking spray and line with a parchment paper.
Place the butter and chocolate in a microwave safe bowl and heat in the microwave until they are melted, stirring every 15 seconds.
Once the butter and chocolate chips is completely melted, add in sugar and whisk well.
Add in the eggs one after another and mix very well after adding each egg.
Add in the flour and stir until no lumps of flour is visible.
Pour half of the brownie batter into the prepared baking pan and top with half of the Oreo pieces.
Pour the remaining batter on the Oreos and top with the rest of the brownies.
Bake in the oven for 25 minutes until a toothpick inserted in the middle comes out with few fudgy crumbs.
Let the brownies cool completely and then slice them.