Preheat the oven to 350 degrees Fahrenheit / 180 degrees Celsius. Prepare a large 10-inch tube pan. Although most sponge cakes say not to grease the pan, I do grease it very lightly. Then dust it lightly with a bit of cocoa, shaking off the excess. Set aside.

Separate the eggs, putting the yolks into one bowl and the whites into another bowl. Beat the whites until they are nearly stiff. Then add in half the sugar while continuing to beat the whites. Turn off the beaters and set the whites aside.

Beat the yolks until they are thick. Add the remaining sugar and beat until the yolks are creamy. Add in the cocoa and potato starch and beat just another minute or two until they are incorporated. Turn off the beaters.

Fold both mixtures together using a rubber spatula. While folding, if the batter seems too liquidy, you may add in a bit more potato starch. Pour this into the prepared tube pan.

Bake for 50 to 75 minutes, checking to make sure it does not burn. When the cake is done, remove it from the oven and place on a wire rack. Leave it for 15 minutes. I like to let it rest on its side, not upside down, while cooling. Then prepare a large piece of foil, lined with one or two paper towels on it.

After the cake has cooled, slide a long, sharp knife around its edges, as well as around the innermost edges. Carefully flip the cake pan over onto the prepared foil. Remove the pan. Leave the cake to sit until it has cooled more, then wrap it up gently and freeze until the day of use.