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1
Heat your oven to 375u00b0 F. Toast hazelnuts on a sheet pan until they start to brown (about 5 to 7 minutes). Be careful-they burn very fast. Place hot hazelnuts inside a dish towel. Gather up all sides of the towel and seal tightly with a rubber band, making a little package (like a beggar's purse). Cool for a few minutes, then aggressively roll the hazelnuts over each other for about 5 minutes. Take a peek. You want to remove about 3/4 of the skins. Pulverize in the food processor until the chunks are about the size of grains of rice. A bit of nut powder is fine.
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2
Grease your cast-iron pan with butter or cooking spray. (It's not necessary to add flour.) Set aside.
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3
Whisk together flour and salt. Set aside.
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4
Place the butter in a medium pan over medium heat. Once it's melted, don't walk away. Take it off the heat once the sizzling stops (4 to 5 minutes) and the brown bits drop down to the bottom of the pan. Pour into the bowl of your standing mixer. Whisk in the brown sugar, and set aside to cool (this takes at least 30 to 40 minutes, maybe more).
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5
Once cool, mix on high speed until it gets light and fluffy (about 3 minutes). Scrape down the side. Add one egg. Beat for 5 seconds on medium speed. Scrape down the sides. Add the other egg. Beat for 5 seconds. Scrape down the sides. Add the vanilla. Beat on low for 8 seconds. Scrape down the sides.
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6
Add 1/3 of the flour/salt mixture. Mix on low speed for 5 seconds. Scrape down the sides. Add the second 1/3 of the flour mixture. Mix on low speed for 5 seconds. Scrape down the sides. Add final 1/3 of flour plus 1/2 of the hazelnuts. Mix on low speed until the last trace of flour disappears. Don't over-mix.
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7
Warm the Nutella on very low heat in a pan or the microwave until it's a bit looser and easier to spread.
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8
Scrape dough into greased pan. Spread out evenly with a spatula. Pour the warm Nutella all over the top. Using a butter knife, swirl the Nutella down into the dough. (Be careful not to over-mix it or the dough will get too sweet.) Sprinkle the top with the remaining pulverized hazelnuts.
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9
Bake for 15 to 18 minutes. It should be light brown all over but still quite soft in the center. That's what you want-it will continue to cook quite a bit as it cools in the pan.
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10
Cool for at least 20 minutes. Then slice and eat. It's good for a few days at room temperature if it's stored in an airtight container. Or it freezes well for a few months.
