01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, allowing extra overhang for lifting the bars out easily.
02 - In a medium mixing bowl, whisk together flour, cocoa powder, baking powder, and salt. Set aside.
03 - In a large bowl, whisk together melted butter and granulated sugar until smooth. Add eggs, one at a time, whisking thoroughly after each. Stir in vanilla extract until fully incorporated.
04 - Gradually fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing to prevent dense brownies.
05 - Fold in the roughly chopped Oreo cookies and semi-sweet chocolate chips, reserving a handful of Oreos for topping.
06 - Spread the batter evenly into the prepared pan. Gently press reserved Oreo pieces onto the surface for a decorative finish.
07 - Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs, indicating a fudgy center. Avoid overbaking.
08 - Allow brownies to cool completely in the pan. Lift out using the parchment overhang and cut into 16 bars.