Sift together flour and cornstarch (or cocoa powder). Combine in a medium size bowl in the following order: yolks, ½ cup sugar, flour, cornstarch or cocoa powder, salt and vanilla.
Whip egg whites with cream of tartar until stiff.
Fold carefully egg whites into yolks.
Line a jelly roll pan 19 x 12 inches (40 x 25 cm) with wax paper. Butter and flour wax paper.
Pour batter into pan. Spread evenly with a spatula.
Preheat oven 325 °F (170 °C). Bake in oven for 10 to 12 minutes or until cake is golden and tester comes out clean. If cake is not golden, place in upper rack and broil for a few minutes.
Moist a kitchen towel, the size of the pan, and place it on a table. Invert the roll onto the kitchen towel and peel off wax paper carefully.
Spread cake with manjarblanco and roll immediately by lifting it with the towel. Let the cake cool completely.
Transfer jelly roll to a platter, seam side down and trim ends.
Dust with sifted confectioners’ sugar or chocolate fudge.