Preheat your oven to 325°F (163°C). Line an 8x8-inch light-colored metal baking pan with foil or parchment paper, allowing some overhang for easy removal. Lightly coat with nonstick cooking spray.

In a microwave-safe bowl, combine the butter and granulated sugar. Microwave for about 1 minute, or until the butter is fully melted. Stir to blend the mixture.

Whisk in the eggs and egg yolk. Whisk vigorously for 1 full minute to create a glossy, smooth texture.

Next, whisk in the vanilla extract, followed by the vegetable oil and unsweetened cocoa powder until well incorporated.

Using a rubber spatula, gently fold in the all-purpose flour, cornstarch, and salt until no dry streaks remain. Be careful not to overmix.

Stir in the chocolate chips to distribute evenly throughout the batter.

Pour the batter into the prepared pan and smooth out the top.

Bake for 30 minutes, or until the brownies are set and a cake tester or toothpick inserted into the center comes out with moist crumbs attached. Avoid overbaking.

Allow the brownies to cool completely in the pan before lifting them out and slicing into squares.

Warm = Gooey texture

Room temperature = Perfectly chewy

Chilled = Fudgy and dense

Store in an airtight container at room temperature for up to 3 days. For longer storage, keep in the fridge for up to 1 week or freeze for up to 3 months.