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Line an 11-by-17-inch baking sheet with parchment paper and set aside.
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In the top of a double boiler, melt chocolate, stirring constantly.
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Stir hazelnut extract and Nutella into the melted chocolate. Remove from heat, and pour the mixture onto the prepared baking sheet; spread evenly. Sprinkle with chopped hazelnuts.
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Chill until firm, 25 to 30 minutes. Break into pieces, and serve. Store in an airtight container in the refrigerator for up to one week.