Preheat oven to 350 degrees F. Line a 15x10-inch jelly roll pan with waxed paper. Grease and flour lined pan; tap out excess. In a medium bowl, combine flour, cocoa powder, cornstarch, baking soda, baking powder and salt. Mix well. In a separate bowl, using an electric mixer set on medium speed, beat egg yolks and 1/4 cup sugar until fluffy.
In a small bowl, using clean beaters, beat egg whites on high until foamy. Gradually add 1/2 cup sugar, beating until stiff, but not dry, peaks form. Fold 1/3 beaten egg whites into egg yolk mixture. Alternately fold in remaining whites and flour mixture. Pour batter in pan; smooth top. Bake until a toothpick inserted in center comes out clean, about 15 minutes.
Dust a clean cloth with remaining sugar. Turn cake out onto prepared cloth; remove waxed paper. Trim the cake's edges. Starting with a long side, tightly roll up cake with cloth. Transfer cake, seam-side down, to a wire rack to cool.
Unroll cake; remove cloth. Spread whipped topping over cake to within 1/2 inch of edges. Re-roll cake; place seam-side down on a plate. Dust with confectioners' sugar before serving.