Prepare Your Pan and Oven: Preheat the oven to 325°F (165°C). Line an 8x8-inch square metal baking pan with parchment paper or foil, ensuring there’s some overhang for easy brownie removal. Lightly coat with nonstick spray to prevent sticking.
Melt Butter and Sugar: In a microwave-safe mixing bowl, combine butter and sugar. Heat for about 1 minute, or until the butter is completely melted. Stir until the sugar begins to dissolve, creating a glossy mixture.
Whisk in Eggs and Flavorings: Add the eggs and the extra yolk to the butter-sugar mixture. Whisk vigorously for 1 full minute, incorporating air for a crackly brownie top. Mix in the vanilla extract until well combined.
Incorporate Oil and Cocoa: Pour in the vegetable oil, followed by the cocoa powder. Stir until a rich, dark chocolate mixture forms, ensuring no dry cocoa lumps remain.
Add Dry Ingredients: Using a rubber spatula, gently fold in the flour, cornstarch, and salt. Mix only until the batter is combined; avoid overmixing to keep the brownies soft and chewy.
Stir in Chocolate Chips: Fold the chocolate chips evenly throughout the batter, adding pockets of melted chocolate in every bite.
Bake to Perfection: Spread the batter evenly into your prepared pan. Bake for 28–32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). Avoid overbaking, as this can dry out the brownies.
Cool and Slice: Allow brownies to cool completely in the pan before lifting them out using the parchment overhang. Slice into 9 large or 16 smaller pieces. Enjoy warm for gooey brownies, at room temperature for chewy texture, or chilled for a fudgier bite.
