Preheat the oven to 180°C (350°F) and grease a square baking pan (approximately 8×8 inches) or line it with parchment paper.
In a heatproof bowl, melt the dark chocolate and butter together. You can do this over a double boiler or in the microwave in 30-second intervals, stirring until smooth. Allow it to cool slightly.
In a separate bowl, whisk together the sugar and eggs until well combined.
Gradually add the melted chocolate mixture to the egg mixture, stirring continuously.
Sift in the plain flour, cocoa powder, baking powder, and a pinch of salt. Gently fold these dry ingredients into the chocolate mixture until no lumps remain.
Pour half of the brownie batter into the prepared baking pan and spread it evenly.
Place the whole Oreo cookies on top of the brownie batter in a single layer.
Pour the remaining brownie batter over the Oreos, covering them completely.
Use a knife or spatula to gently swirl the top layer of brownie batter with the Oreos. This will create a marbled effect.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (but not wet batter).
Allow the Oreo Brownies to cool in the pan for about 10-15 minutes before transferring them to a wire rack to cool completely.
Once cooled, cut the brownies into squares and serve. Enjoy your delicious Oreo Brownies!
