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Preheat the oven to 350 degrees. Spray a 9-inch square baking pan with cooking spray and set aside.
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Combine the butter and sugars in a large mixing bowl and beat vigorously with a wooden spoon until smooth. Mix in the egg and vanilla until combined.
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Mix in the salt, then the flour and stir until the flour is just incorporated. Fold in the chocolate chips until evenly distributed.
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Pour 1/4 cup of the Nutella over the top of the batter and stir with a rubber spatula ONE TIME. You’re not trying to mix it in fully, but to create ribbons.
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Scrape the dough into the prepared pan and smooth the top. Dollop the remaining Nutella over the dough and use a knife to swirl the two together.
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Bake 22 to 26 minutes, until golden around the edges but not quite set in the middle. (The middle should still jiggle a little bit if you tap the pan.)
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Remove from the oven and cool completely on a wire rack. Slice into bars and serve.