In the bowl of an electric mixer, beat egg whites, slowly adding in the sugar. Once all sugar has been added, raise speed to high and beat until stiff peaks form.
Lower speed and slowly add yolks, oil, potato starch, and baking powder.
Pour ¾ of the batter into 3 loaf pans.
Add cocoa to the remaining batter in mixer and mix gently. Pour into the center of batter in pan and marbleize with fork.
Bake loaves for 40 minutes (a 9x13 will take 1 hour). Note that if your Pesach oven bakes differently than your usual oven, it might take a little longer. The cake falls in a bit, but is still delicious!