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1
Melt chocolate with Nutella and keep warm.
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2
Beat cream with an electric mixer until it just holds soft peaks, then chill.
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3
Beat egg and yolks in cleaned mixer bowl with cleaned beaters at high speed until thick and pale, about 5 minutes.
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4
Meanwhile, bring sugar, water, and corn syrup to a boil in a 1-qt heavy saucepan over medium heat, stirring occasionally until sugar has dissolved. Continue to boil, undisturbed, until candy thermometer registers 238u00b0F (soft-ball stage; you will need to tilt pan to get temperature), about 4 minutes.
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5
Reduce mixer speed to medium and add hot sugar mixture to eggs in a slow steam (try to avoid beaters and side of bowl), then beat until mixture is pale, thick, tripled in volume, and completely cool, about 5 minutes.
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6
Fold chocolate mixture into egg mixture. Fold in one third of whipped cream to lighten, then fold in remaining whipped cream gently but thoroughly. Chill at least 4 hours.
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7
Line a baking pan with foil (shiny side up).
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8
Heat sugar in a heavy 10-inch skillet over medium heat, stirring with a fork to heat sugar evenly, until it starts to melt, then stop stirring and cook, swirling skillet occasionally so sugar melts evenly, until it is dark amber. Add hazelnuts, stirring until coated. Transfer to foil and cool completely, then coarsely chop.
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9
Preheat oven to 400u00b0F with rack in middle. Line a large heavy baking sheet with parchment paper.
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10
Roll out puff pastry onto a lightly floured surface with a lightly floured rolling pin into a 17- by 10-inch rectangle (about 1/8 inch thick). Transfer to baking sheet, then cover with a sheet of parchment and top with a heavy baking sheet or sheet pan to weight down. Bake until pale golden, 15 to 20 minutes.
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11
Meanwhile, stir together corn syrup and water.
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12
Remove top baking sheet and parchment. Generously brush pastry with corn syrup mixture and cover with parchment, then invert pastry and brush other side with corn syrup mixture. Replace parchment and baking sheet and bake until deep golden and crisp, 15 to 20 minutes more. Cool completely.
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13
Using a serrated knife, trim edges of pastry, then halve lengthwise and cut each half into 8 equal rectangles for a total of 16 rectangles.
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14
Arrange 8 rectangles on a baking sheet and dust with confectioners' sugar to cover. (To mark pastry decoratively, see cooks' note, below.)
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15
Dab a small amount of hazelnut creme in center of 8 plates, then put an undecorated pastry rectangle on top. Divide remaining hazelnut creme among pastry rectangles. Sprinkle with some of the candied hazelnuts and top with decorated pastry rectangles. Sprinkle with more candied hazelnuts.
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16
Hazelnut creme can be chilled up to 2 days.
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17
Candied hazelnuts keep in an airtight container 3 weeks.
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18
Pastry can be baked 2 days ahead and kept in an airtight container at room temperature.
