Heat oven to 375.
Line the bottom a 15x10x1 baking or jelly roll pan with parchment paper rectangle cut 1/2 inch smaller than the pan (14×9).
Grease the exposed edges of the pan.
Sift the cake flour, cocoa powder, malted milk powder, baking powder, and baking soda into a bowl.
In a mixer, beat the 4 eggs yolks until thick and yellow.
Add the 1/4 cup sugar and beat on high until dissolved.
Transfer to a bowl and set the mixture aside.
Wash the mixer bowl and beater and dry completely.
In your clean mixing bowl beat the egg whites on medium until soft peaks form.
By the tablespoon, add the 1/2 cup sugar and beat until stiff peaks form.
Gently fold the egg yolk mixture into the beaten egg white mixture.
Quickly, but gently, by the tablespoon, alternate adding in the flour mixture and the water, just incorporating with a spatula. If it looks a little marbled, that's okay.
Gently spread the marshmallowy batter as evenly as you can across the prepared pan.
Bake for 12-15 minutes, until the centre of the cake springs back when lightly pressed with a finger.
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. [The cake may look a bit bubbly or even mountainous! No worries! It will flatten out with the folding, anyway!]
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 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.]