In a large bowl, combine the sugar, cocoa and yogurt. Stir in egg whites and vanilla. Gradually stir in flour and nuts just until combined.
Pour into an 8-in. square baking pan that has been coated with cooking spray. Bake at 350° for 25-28 minutes or until a toothpick inserted near the center comes out clean. Dust with confectioners' sugar. Yield: 16 servings.