Preheat oven to 325 degrees.

In a large bowl, mix together warm melted butter and sugar.

As you are mixing, whisk in the two whole eggs one at a time, adding the egg yolk last.

Stir in the vanilla extract and vegetable oil.

Once incorporated, mix in the cocoa powder.

Using a spatula, fold in the flour, baking soda, cornstarch, and salt until combined.

Tips: It's ok if you can still see some flour, you don't want to over mix which will lead to tough brownies.

Gently fold in the chocolate chips.

Transfer the brownie batter to a 9x9 baking pan sprayed with nonstick cooking spray. Top with additional chocolate chips and bake in the oven for 30 minutes, or until the edges are set and a cake test is successfully performed at the center of the brownies.

Tip: A cake test is when you insert a toothpick at the center of the pan, and it comes back clean or with pieces of cake attached. If there is any uncooked batter left on the toothpick, return to oven for 5 additional minutes. Be careful not to overcook it at this point, since the brownies will continue to cook as they cool.

Let cool completely before cutting and serving.

They will last for up to a week in an airtight container.

Enjoy!