Put the hazelnuts, cocoa powder and 1 cup of the confectioners' sugar in the bowl of a food processor and process until the nuts are very finely ground and powdery, about 3 minutes. Sift with a medium-mesh strainer and discard any pieces of nuts that didn't pass through the strainer.
Beat the egg whites and salt in a large bowl using an electric mixer on medium-high speed until they hold firm but not dry peaks, about 2 minutes. While beating, gradually add the remaining 1 cup confectioners' sugar, scraping down the sides of the bowl as needed until the peaks are firm and shiny. Fold the nut mixture into the egg whites with a large rubber spatula until just blended. Transfer to a large piping bag fitted with a 1/2-inch plain tip. Pipe 32 two-inch rounds on the prepared pans. Set aside to air-dry for 30 minutes.
Preheat the oven to 325 degrees F.
Bake until the tops of the cookies are puffed and set, 18 to 20 minutes. Take care not to overcook the cookies or they will crack. Cool the cookies for 5 minutes, then gently peel from the parchment and transfer to a wire rack to cool completely.
Meanwhile, put the chocolate in a medium heatproof bowl. Heat the cream to a boil in a small saucepot over medium heat. Pour the cream over the chocolate and set aside until the chocolate is soft, about 5 minutes. Whisk until smooth; let stand at room temperature until thick but still spreadable. Sandwich about 1 teaspoon of the chocolate mixture between two cookies and repeat with the remaining filling and cookies.
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