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1
To make the Nutella chunks: Add the Nutella and butter to a small microwave-safe bowl. Heat in 30 second bursts on 50% power, stirring in between, until the mixture is smooth and glossy. (Mine needed 1 minute and 30 seconds total.) Spread the Nutella mixture into a rectangular shape on a parchment-lined baking sheet - it should be about 1/4-inch thick. Transfer to the freezer for at least 2 hours, or until firm. Place the Nutella rectangle on a cutting board and, with a sharp knife, cut into small chunks. Try not to touch them any more than you need to. Once chopped, place them in a bowl and put back in the freezer until you're ready to use them.
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2
To make the cookie bars: Preheat oven to 325 F with a rack in the lower third. Spray a 13x9 baking dish with nonstick cooking spray then line the dish with aluminum foil, leaving an overhang on opposite sides to allow you to lift the bars out after baking. Spray the aluminum foil with nonstick cooking spray too.
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3
In a medium bowl, whisk the flour, baking soda and salt together. In a second bowl, combine the butter and both sugars, whisking until combined. Add the egg, egg yolk and vanilla and whisk to completely incorporate. Add the dry ingredients to the wet, and use a rubber spatula to fold the dry into the wet just until combined. (The batter will be quite thick.) Remove the Nutella chunks from the freezer and add them to the dough. Use the rubber spatula to fold in the chunks, working quickly to distribute them as best you can.
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4
Transfer the batter to the prepared pan, spreading into an even layer in the bottom. Bake for 27-30 minutes, or until the bars are golden brown and slightly firm to the touch. The bars will begin to pull away from the sides of the pan when they're done.
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5
Remove the pan to a wire rack and let the bars cool completely before lifting them out of the pan and cutting into squares to serve.
