Preheat the oven to 350 degrees. Line a 9x9 metal baking pan with parchment paper.
Separate Oreo cookies. Place 25 Oreos in a large ziplock bag and crush into crumbs (or use a food processor). Reserve 16 Oreos whole. Break remaining 5 Oreos into pieces.
Place one and a quarter cup of the dark chocolate chips into a medium microwave safe bowl (reserve the remaining chips for later). Add the butter and microwave 30 seconds, then stir well. Microwave again if needed until completely melted.
Pour chocolate mixture into a mixing bowl. Add the sugar, eggs, and vanilla and beat until combined.
Add the flour, baking powder and salt and beat to combine. Then switch to using a wooden spoon and stir in the Oreo crumbs until combined.
Press half of the brownie batter into the lined 9x9 inch pan. Place 16 whole Oreo cookies over the batter and press them down. Spread the remaining brownie batter out into the pan. Top with the broken Oreo cookie pieces and the reserved dark chocolate chips, and press those down.
Bake for approximately 20 minutes, checking the brownies a few minutes earlier or as soon as you can smell them. A toothpick inserted near the center of the brownies should come out with moist crumbs attached - just remember you'll need to poke it a couple times to find a place where an Oreo doesn't block the toothpick. (If it comes out with wet batter on it they need a few more minutes, if it comes out clean you've overcooked them a bit.)
Cool brownies in the pan completely, at least four hours, preferable six to eight hours. Use the parchment paper to removed the brownies from the pan and slice into 16-20 pieces.