Preheat oven to 325°F. Spray a 9X9-inch square baking pan with nonstick spray. Place parchment paper on the pan's bottom and sides. Spray again.
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Mix flour, cocoa powder, cornstarch, and salt in a medium basin. Set aside.Add sugar, powdered sugar, eggs, and egg yolk to a whisk-attached mixer. Beat fast for 5 minutes.
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Mix in melted butter, oil, and vanilla. Mix slowly until mixed. Add dry ingredients and mix slowly.Take the mixer out and stir the batter with a rubber spatula. Add Nutella and stir.
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Spread batter in pan. Heat Nutella in a microwave-safe bowl for 30 seconds. Scoop Nutella into brownie batter with a 1 teaspoon. Swirl Nutella using a butter knife.
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Bake 40-50 minutes. When a toothpick inserted comes out with wet crumbs, brownies are done. cold brownies in the pan on a cooling rack until totally cold.
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