Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper or lightly grease it and dust with cocoa powder.

In a microwave-safe bowl or on the stovetop, melt the butter. Stir in the granulated sugar until it looks glossy and well-combined.

Let the butter-sugar mixture cool slightly, then whisk in the eggs one at a time. Add the vanilla and whisk until smooth.

Sift the cocoa powder, flour, salt, and baking powder into the wet mixture. Gently fold until mostly combined.

Fold in the chocolate chips and most of the chopped Oreos, reserving some for topping.

Spread the batter evenly in your prepared pan. Top with reserved chopped Oreos and a few whole Oreos for decoration.

Bake for 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Start checking at 22 minutes.

Let the brownies cool completely on a wire rack before lifting the parchment to remove them from the pan.

Slice into squares. Store in an airtight container at room temperature for up to 3 days.