Baci Di Dama (Hazelnut Meringue Sandwich Cookies)

Heat oven to 325°. Place hazelnuts, ½ cup sugar, and salt in a food processor, and process until nuts are finely ground. Transfer to a bowl, and stir in cornstarch; set aside. Beat egg whites in a large bowl on high speed of a mixer until soft peaks form; slowly add remaining sugar, and beat until stiff peaks form. Add hazelnut mixture, and gently fold with a rubber spatula until evenly combined. Transfer to a piping bag fitted with a ⅜" plain tip, and pipe about 3 dozen ½" mounds of batter onto parchment paper-lined baking sheets. Bake until dried and lightly browned, about 16 minutes. Dip the bottoms of half the cookies with chocolate and sandwich with remaining plain cookies; let chocolate set before serving, about 5 minutes.