Makes 36 cookies
1/2 cup whole hazelnuts, shelled
3/4 cup powdered sugar, divided
1/2 teaspoon finely grated lemon zest
3 large egg whites
3/8 teaspoon cream of tartar
1/4 cup plus 2 tablespoons superfine sugar, or
regular sugar pulverized in a food processor
Preheat oven to 350 degrees F.
Place the hazelnuts on a cookie sheet and roast for 15 to 20 minutes, until the skins crack. Remove from oven and rub the nuts briskly between your hands or in a kitchen towel until most of the skins are removed. Place the nuts in a food processor with 1 tablespoon of the powdered sugar and the lemon zest and finely grind.
In a medium bowl, beat the egg whites until frothy, add the cream of tartar and beat at medium speed while gradually adding 1 tablespoon superfine sugar. Beat until soft peaks form when beaters are raised. Add 1 tablespoon superfine sugar and increase mixer speed to high. When stiff peaks form when the beaters are raised, gradually beat in the remaining superfine sugar. Continue beating until whites are very stiff and glossy. Sift the remaining powdered sugar over the meringue and fold it in with a wire whisk.
Line two cookie sheets with aluminum foil. Place 1 cup of the meringue mixture in a small bowl and stir in the ground nuts. Place the remaining meringue in a pastry bag with a No. 12 tube or in a reclosable quart-size freezer bag with a small corner snipped off. Squeeze the meringue mixture onto the foil in 2-inch heart shapes, leaving a small opening in the center of each heart. Leave about 1 inch between each cookie. When you've used all the plain meringue, fill the bag with the hazelnut meringue and squeeze that mixture into the center of each heart, using enough filling to slightly expand the cookie.
If the cookies need any reshaping, moisten a finger in cold water and shake off the excess before touching the meringue, or it will stick to your finger.
Let the cookies dry at room temperature until set, about 1 hour. Then preheat the oven to 200 degrees F and bake the cookies 1 hour. Turn off the oven and leave cookies in overnight, then peel from foil. Or remove immediately from oven and use a metal spatula to remove from foil, then cool on a wire rack. Store cookies in an airtight container.