Heat oven to 350 degrees F. Coat 15 x 10 x 1-inch jelly roll pan with nonstick cooking spray. Line with waxed paper; coat paper. Dust a clean kitchen cloth with 2 tablespoons of the confectioners' sugar.

Sift together cocoa, flour, baking powder and salt; set aside.

In a medium-size bowl, beat egg yolks, 1/3 cup of the granulated sugar and 1/2 teaspoon of the vanilla on medium speed about 1 minute until well blended. Reduce speed to low and add in cocoa mixture; set aside.

In another clean medium-size bowl, beat egg whites and cream of tartar on medium speed until foamy. Increase speed to high; gradually beat in remaining 1/3 cup granulated sugar, about 3 minutes, until stiff peaks form. Fold one-third of the whites into egg mixture; then fold in remaining whites.

Pour batter into prepared pan, spreading evenly.

Bake cake at 350 degrees F for about 23 minutes or until cake springs back when lightly pressed. Cool in pan on rack for 5 minutes. Then, invert cake onto sugar dusted cloth. Remove pan and waxed paper and dust with 2 tablespoons confectioners' sugar. Beginning at a short end, roll up cake, enclosing towel. Place cake, seam side down on a cooling rack. Cool completely.

In a medium-size bowl, beat heavy cream and remaining 2 tablespoons confectioners' sugar and 1/2 teaspoon vanilla on high speed about 3 minutes until stiff peaks form. Unroll cake and spread with whipped cream.

Carefully roll up cake without towel. Dust with additional confectioners' sugar before serving.