Place egg whites in a small bowl; let stand at room temperature for 30 minutes., Preheat oven to 350°. Place hazelnuts and confectioners' sugar in a food processor; pulse until nuts are finely ground., Add salt to egg whites; beat on medium speed until soft peaks form. Gradually add superfine sugar, 1 tablespoon at a time, beating on high until stiff peaks form. Fold in hazelnut mixture., Transfer mixture to a pastry bag fitted with a round tip. Pipe 1-in.-diameter cookies 2 in. apart onto parchment-lined baking sheets. Bake until lightly browned and firm to the touch, 9-12 minutes. Transfer cookies on the parchment to wire racks; cool completely., For the buttercream, in a heavy saucepan, combine sugar and water. Bring to a boil; cook over medium-high heat until sugar is dissolved. Remove from heat. In a small bowl, whisk a small amount of hot syrup into egg yolks; return all to pan, whisking constantly. Cook until thickened, 2-3 minutes, stirring constantly; remove from heat. Stir in espresso powder and vanilla; cool completely., In a stand mixer with the whisk attachment, beat butter until creamy. Gradually beat in cooled syrup. Beat in confectioners' sugar until fluffy. Refrigerate until mixture firms to a spreading consistency, about 10 minutes., Spread about 1-1/2 tsp buttercream onto the bottoms of half the cookies; top with remaining cookies. Store in airtight containers in the refrigerator.