Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper or foil, leaving enough overhang to lift the brownies out later. Lightly grease the parchment.

In a medium saucepan or microwave-safe bowl, melt the butter. Let it cool slightly, then whisk in the sugar until glossy.

Whisk the eggs in one at a time, mixing well after each. Stir in the vanilla until smooth.

In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder. Gradually fold the dry mixture into the wet mixture.

Fold in the semisweet chocolate chips and the coarsely chopped Oreos, leaving a bit of the chopped cookies for the top.

Spread the batter evenly into the prepared pan. Press the reserved whole Oreos gently into the top.

Bake for 25–30 minutes. You want the edges set and the center slightly soft. A toothpick inserted near the middle should come out with a few moist crumbs.

Cool the brownies completely in the pan on a wire rack, about 30 minutes.

Use the parchment overhang to lift them out and slice into 16 squares.