Begin by preheating the oven to 180°C (350°F) and either grease an 8×8 inch square baking pan or line it with parchment paper.

In a heatproof bowl, combine the dark chocolate and butter, and melt the mixture either using a double boiler or by microwaving it in 30-second increments until smooth. Allow it to cool slightly.

In a separate bowl, blend the sugar and eggs until thoroughly mixed.

Gradually incorporate the melted chocolate mixture into the egg mixture, stirring continuously.

Sift in the plain flour, cocoa powder, baking powder, and a pinch of salt. Carefully fold these dry ingredients into the chocolate mixture until there are no lumps.

Pour half of the brownie batter into the prepared baking pan, spreading it evenly.

Arrange the whole Oreo cookies on top of the brownie batter in a single layer.

Pour the remaining brownie batter over the Oreos, ensuring they are completely covered.

Use a knife or spatula to gently swirl the top layer of brownie batter with the Oreos to create a marbled effect.

Bake the pan in the preheated oven for 25-30 minutes or until a toothpick inserted into the centre comes out with a few moist crumbs, but not wet batter.

Allow the Oreo Brownies to cool in the pan for approximately 10-15 minutes before transferring them to a wire rack to cool completely.

Once they have cooled, cut the brownies into squares and serve. Enjoy your scrumptious Oreo Brownies!