Preheat the oven to 325°F. Line a 8 by 8-inch pan with foil and spray with nonstick cooking spray.
In a microwave safe bowl, add the butter and sugar. Microwave for about 1 minute, or until the butter is melted. Whisk in the eggs, egg yolk, and vanilla. Stir in the oil and cocoa powder.
With a rubber spatula, stir in the flour, baking soda, cornstarch, and salt until combined. Stir in the chocolate chips.
Spread the brownie batter evenly into the prepared pan. Place in the oven and bake for 30 minutes, or until the brownies are set and a cake tested inserted into the center has moist crumbs attached. Do not overcook. Let cool completely before cutting and serving.
Brownies can be stored in an airtight container at room temperature for up to 3 days.