Beat egg yolks with vanilla. Blend cocoa with water, beat into egg yolk mixture. Sift flour with baking powder. Gradually add dry ingredients, mixing until smooth. Beat egg white with salt until soft peaks form. Gradually add sugar, beating until stiff peaks form. With spatula, fold egg whites into chocolate mixture. Pour batter into greased and floured waxed paper lined 10 x 15 inch jellyroll pan. Bake at 325 degrees for 15 minutes. Run knife around edge. Turn hot cake onto cloth heavily dusted with powdered sugar. Remove waxed paper. Starting from narrow end, roll up cake with cloth and let cool. When cake has cooled, roll cake back out and spread on softened chocolate (or vanilla) ice cream. Reroll and freeze until solid. Slice and serve with chocolate syrup poured over each slice.
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