Line an 8x8-inch baking pan with parchment paper. Spoon the Nutella onto the parchment paper and spread it into an even layer. Place into the freezer to chill for 1 hour.
13 ounces Nutella
Once the Nutella has chilled, preheat oven to 350°F. In a medium bowl, whisk the flour, salt, baking powder, and baking soda together.
1 cup all-purpose flour, 1 teaspoon kosher salt, ¼ teaspoon baking powder, ¼ teaspoon baking soda
In a large bowl, whisk the butter, oil, brown sugar, and sugar together. Add in the eggs and egg yolks and whisk. Add in the cocoa powder and stir together. Lastly, add in the flour mixture and stir just to combine.
½ cup unsalted butter, ½ cup vegetable oil, ¾ cup brown sugar, ⅓ cup granulated sugar, 2 large eggs, 2 large egg yolks, ⅔ cup unsweetened cocoa powder
Remove the Nutella from the baking pan, leaving the parchment paper. Pour in half of the brownie batter. Place the nutella slab on top of the brownie batter. Pour the rest of the brownie batter on top of the nutella.
Place into the preheated oven to bake for 45-55 minutes, or until a toothpick inserted comes out clean or with moist crumbs.
Once baked, remove from the oven to cool for a few minutes before cutting and serving.