Heat oven to 375f.
Grease a 15x10x1 baking or jelly roll pan.
Line it with a waxed paper rectangle cut 1/2 inch smaller than the pan (14×9).
Grease the paper.
Sift flour, cocoa, baking powder, baking soda, and salt into a bowl or onto another sheet of waxed paper.
In a mixer or in a bowl, by hand, beat eggs yolks and vanilla until thick and yellow.
Add the 1/4 c sugar and beat on high until dissolved.
Set the mixture aside in a clean bowl. Wash the mixer bowl and beaters.
Beat the egg whites on medium until soft peaks form.
Add the 1/2 cup sugar slowly and beat until stiff peaks form.
Fold the egg yolk mixture into the beaten egg white mixture quickly with the mixer or with a spatula.
Quickly, but gently, by the quarter cup, alternate adding in the flour mixture and the water, just incorporating with the mixer or a spatula.
Spread batter into the pan as evenly as possible. It is best to smooth the top of the cake as much as possible with a large butter knife or icing spreader.
Bake 12-15 minutes until the centre springs back when pressed lightly with a fingertip. [The cake may look a bit bubbly or even mountainous! No worries! It will flatten out with the folding, anyway!]
While the cake is baking, spread a large tea towel on a countertop or table and dust the towel with icing sugar.
Also, make sure your mixing bowl and beaters are clean. And place them in the refrigerator or freezer to chill for your whipped cream.
Take the cake out of the oven, place the pan on a wire rack, and let it cool only slightly, for 3-5 minutes.
Run a knife along all four edges of the pan to loosen the cake.
While still warm, invert the cake onto the sugar-dusted towel and gently remove the parchment or waxed paper from the surface of the cake.
Make sure that the cake is situated in "landscape" mode in front of you, so that you will maintain its 15-in width and that the 10-inch sides will "roll" into a "log."
Gently roll the cake right in towel, dusting with icing sugar as you go.
Set the rolled cake in its towel on a flat surface or rack and cool completely. [You can store the cake like this for up to one day or re-roll the cake in wax paper, wrap wax-paper-rolled cake in foil and freeze for 2-3 weeks.]