Step 1

Line 2 baking sheets with parchment paper.

Step 2

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.

Step 3

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.

Step 4

Preheat the oven to 325 degrees F.

Step 5

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.

Step 6

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.