1. Preheat the oven:
Preheat your oven to 180°C (350°F). Grease and line a 9x9-inch (23x23 cm) square baking pan with parchment paper, leaving some overhang for easy removal.

2. Melt the chocolate and butter:
In a heatproof bowl, melt the dark chocolate and butter together. You can do this in the microwave in 20-second intervals or use a double boiler. Stir until smooth and set aside to cool slightly.

3. Mix the wet ingredients:
In a large mixing bowl, whisk together the granulated sugar and brown sugar. Add the eggs, one at a time, whisking well after each addition. Stir in the vanilla extract.

4. Combine wet and dry ingredients:
Pour the slightly cooled chocolate mixture into the sugar and egg mixture. Stir until well combined.

5. Add the dry ingredients:
In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until just combined.

6. Assemble the brownies:
Pour half of the brownie batter into the prepared baking pan, spreading it evenly. Place a layer of Oreos on top of the batter. You can break some of the Oreos in half if you prefer more surface area. Pour the remaining brownie batter over the Oreos and spread it evenly to cover them.

7. Bake the brownies:
Bake in the preheated oven for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake, as you want the brownies to stay fudgy.

8. Cool and cut:
Allow the brownies to cool completely in the pan before cutting them into squares. If desired, sprinkle the top with some crushed Oreo pieces for added texture and flavour.