Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a medium saucepan, melt the butter over low heat. Remove from heat.
Whisk the granulated sugar and brown sugar into the melted butter until combined.
Whisk in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the cocoa powder, flour, salt, and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Gently fold in the chopped chocolate.
Pour half of the batter into the prepared pan and spread evenly.
Arrange half of the Oreo cookies over the batter.
Pour the remaining batter over the Oreos and top with the rest of the cookies. Gently press them into the top layer.
Bake for 30 to 35 minutes. The edges should look set, and a toothpick inserted near the center should come out with moist crumbs attached, not wet batter.
Let the brownies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang.
Cut into squares and share the joy.
Notes
For the best crinkly top, ensure your dry ingredients are fully incorporated but stop mixing as soon as you no longer see streaks of flour.
Chilling the brownies for at least two hours before cutting makes for cleaner slices.
You can dip the Oreos lightly in water before placing them on top for extra moisture retention, though this is optional.