1. Heat oven to 425°F. Grease bottom of 9-inch square baking pan.
2. Melt butter over low heat. Add cocoa and 1 cup sugar; stir well. Remove from heat; cool about 5 minutes. Stir in flour and vanilla. Add egg yolks, one at a time, beating well after each addition.
3. Beat egg whites with 1 tablespoon sugar until soft peaks form; gradually fold into chocolate mixture. Spoon batter into prepared pan.
4. Bake 18 to 20 minutes or until edges are firm (center will be slightly soft). Cool completely. Refrigerate about 4 hours. Let stand at room temperature about 15 minutes before serving. Garnish with powdered sugar. 20 servings.