Preheat the oven to 340°F (170°C). Grease an 8x8 inch (20x20-cm) square baking tin with oil and line it with baking paper, ensuring there’s an overhang for easy removal later.
Melt the butter and dark chocolate in a heatproof bowl. You can do this over a double boiler or simply microwave it in short bursts, stirring in between. Once melted, stir in the sugar and warm milk until smooth. Whisk until the sugar fully dissolves.
Add the dry ingredients. Sift the flour, cocoa powder, baking powder, and salt into the chocolate mixture. Whisk together until the batter is smooth and well-combined.
Fold in chocolate chips and chopped Oreo cookies, mixing gently to distribute them evenly throughout the batter.
Pour the brownie batter into the prepared tin, smoothing the top with a spatula. Arrange whole Oreo cookies on top for that perfect finishing touch.
Bake for 25-30 minutes, or until the brownies have risen evenly and the center is set. You can check doneness with a toothpick – it should come out with a few moist crumbs.
Sprinkle extra chocolate chips on top while the brownies are still warm for a luscious, melty finish (Optional)
Allow the brownies to cool completely in the tin before cutting them into squares with a warm knife. Enjoy the gooey, indulgent goodness!