
Preheat the oven to 325°F. Line an 8 by 8-inch light-colored metal pan** with foil or parchment paper and spray with nonstick cooking spray.
In a large microwave-safe bowl, melt the butter. Whisk in the sugar. Allow to cool until just warm, then whisk in the eggs and egg yolk, whisking vigorously for 1 minute. Whisk in the vanilla, oil, and cocoa powders.
With a rubber spatula, stir in the flour, cornstarch, and salt until combined. Stir in the chocolate chips and half the Oreo pieces. Spread the brownie batter evenly into the prepared pan. Top with the remaining Oreo pieces, pressing in slightly.
Bake for 30-32 minutes, or until the brownies are set and a cake tester inserted into the center has moist crumbs attached. Let cool completely to solidify before cutting and serving.
Brownies can be stored in an airtight container at room temperature for up to 3 days. Brownies will be gooey when served warm, chewy when served at room temperature, and fudgy when served chilled.