Preheat the oven to 350F. Line an 8 x 8 baking pan with parchment paper.
Mix together the butter, chocolate, granulated sugar and brown sugar in a saucepan and heat on medium, stirring frequently.
Once the chocolate is almost completely melted, remove the pan from heat and stir to allow the chocolate to finish melting. Allow the mixture to cool for about 5 minutes.
Mix together the flour, cocoa powder cornstarch, and salt together in a bowl while the butter mixture cools, then set aside.
Add in the eggs and vanilla to the melted chocolate mixture then beat well for about 3 minutes.
Stir in the flour mixture and mix until just combined and there are no streaks of flour left. Do not overmix.
Add ¾ of the crushed Oreos into the batter.
Pour the batter into the prepared baking pan, then top with the remaining Oreos.
Bake for about 25 - 30 minutes, or until a toothpick comes out of the center with a few moist crumbs attached to it.
Cool for about 5 - 10 minutes before transferring to a wire rack to cool completely. Only cut once the brownies have completely cooled for the best results.
Notes
Store the Oreo brownies in an airtight container for up to a week.
Take care to measure the flour correctly by fluffing it and spooning into your measuring cup or using a kitchen scale and the metric conversions provided