Preheat your oven to 350°F (175°C). Grease your 8x8 inch baking pan or line it with parchment paper for easy removal of the brownies later.
In a microwave-safe bowl, combine the unsalted butter and semi-sweet chocolate. Microwave in 30-second intervals, stirring in between, until fully melted and smooth.
Once the chocolate mixture is cool to the touch, mix in the vegetable oil and sifted powdered sugar. Whisk until well combined and smooth.
Add the eggs and egg yolk one at a time, whisking well after each addition. Stir in the vanilla extract until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, cornstarch, salt, and chocolate chips. Gradually fold this mixture into the wet ingredients until just combined. Be careful not to overmix!
Pour half of the brownie batter into the prepared pan, smoothing it out evenly. Next, dollop the Nutella over the batter, creating a nice line down the center. Use a spatula or knife to swirl the Nutella slightly into the batter.
Pour the remaining brownie batter over the Nutella layer, ensuring it is covered completely. You can use the spatula to smooth the top if needed.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake, as you want that fudgy texture!
Allow the brownies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Once cooled, slice into squares and enjoy!