Grind 1 cup of the confectioners' sugar, the hazelnuts, and cocoa in a food processor until powdery, about 3 minutes. Sift mixture through a medium mesh strainer onto a piece of parchment paper, then discard any large pieces of nuts that didn't come through the strainer.
With an electric mixer, beat the egg whites and salt until they hold firm but not dry peaks, about 2 minutes. Gradually add the remaining 1 cup confectioners' sugar, a tablespoon at a time, scraping down the sides of the bowl as needed; this can take up to 3 minutes. Beat for an extra 30 seconds until the peaks are firm and shiny but still not dry. Fold the nut mixture gently into the egg whites with a large rubber spatula.
Transfer mixture into a large piping bag fitted with a 1/2-inch plain piping tip. Pipe 32 2-inch by about 1/3-inch thick rounds, evenly spaced, on each of the prepared pans. Set aside to air dry for 30 minutes before baking.
Meanwhile make the filling: Put chocolate in a medium heatproof bowl. In a small saucepan, bring cream, with espresso powder mixed in, to a boil.
Pour cream over chocolate and shake bowl gently so cream settles around the chocolate. Set mixture aside until the chocolate is soft, about 5 minutes.
Whisk gently until smooth, taking care not to incorporate too many air bubbles. Cool at room temperature until set up.
Preheat oven to 325 degrees F.
Bake macaroons until set and puffed, about 18 to 20 minutes. Take care not to overcook the macaroons or they'll crack. Cool cookies for 5 minutes then gently peel from paper.
Let cool completely.
Spread 1 teaspoon of mocha filling on a cookie and sandwich 2 cookies together.
