PREPARATION:

1. Place 1 oven rack in the top third of oven and 1 oven rack in the bottom third of oven; preheat oven to 300°F. Line 2 large cookie sheets with aluminum foil, shiny side up.
2. Stir powdered sugar and 1/2 cup walnuts with fork in medium bowl; set aside. Crumble brown sugar into small bowl; set aside.
3. Beat egg whites and salt in large bowl with electric mixer at high speed until foamy. Add cream of tartar; beat 30 seconds or until mixture forms soft peaks. Sprinkle brown sugar, 1 tablespoon at a time, over egg white mixture; beat at high speed until each addition is completely absorbed. Beat 2 to 3 minutes or until mixture forms stiff peaks. Beat in maple extract at low speed. Gently fold in walnut mixture.
4. Drop level tablespoonfuls of dough to form mounds about 1 inch apart on prepared cookie sheets. Sprinkle cookies with remaining 1/3 cup ground walnuts. Bake 25 minutes or until cookies feel dry on surface but remain soft inside. (Rotate cookie sheets from top to bottom halfway through baking time.)
5. Slide foil with cookies onto wire racks; cool completely. Carefully remove cookies from foil. Store in airtight container with waxed paper between layers of cookies. Cookies are best the day they are baked.