In a large bowl, let egg whites stand at room temperature for 30 minutes. Line two large cookie sheets with parchment paper or foil; set aside.

Preheat oven to 300 degrees F. For meringue: Add cream of tartar and salt to egg whites. Beat with an electric mixer on medium to high speed until soft peaks form (tips curl). Gradually add the 2/3 cup powdered sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form (tips stand straight). Gently fold in ground hazelnuts.

Transfer meringue to a pastry bag fitted with a large round tip or to a large plastic bag. If using a plastic bag, cut off a corner. Pipe meringue into 2-inch circles (each about 1/4 inch tall), spacing circles 1 inch apart on the prepared cookie sheets. (Or spoon meringue onto prepared cookie sheets, using a well-rounded teaspoon of meringue for each circle. Spread meringue to 2 inches in diameter.)

Place cookie sheets in preheated oven, placing one on the middle rack and the other on the rack below. Bake for 12 minutes. Turn off oven. Let meringues dry in oven, with the door closed, for 1 hour. Remove from oven; cool completely on cookie sheets. Carefully peel cookies off parchment paper or foil.

Meanwhile, for coffee butter cream: In a small bowl, dissolve coffee crystals in the hot water; set aside. In a large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add the 1-1/4 cups powdered sugar, the coffee mixture, and vanilla; beat until well mixed.

Transfer coffee cream mixture to a pastry bag fitted with a small open-star tip. Pipe cream atop cooled cookies. Place a whole hazelnut atop coffee cream on each cookie. Serve within 1 hour. Makes about 36 cookies.

Layer untopped cookies between waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months. Thaw cookies, if frozen. Top as directed in Step 3.