Step 1

Preheat the oven to 325°F. Line a 8 by 8-inch pan with foil or parchment paper and spray with nonstick cooking spray.

Step 2

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.

Step 3

With a rubber spatula, stir in the flour, baking soda, cornstarch, and salt until combined. Stir in the chocolate chips.

Step 4

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 tester inserted into the center has moist crumbs attached. Do not overcook. Let cool completely before cutting and serving.

Step 5

Brownies can be stored in an airtight container at room temperature for up to 3 days.