How to cook
Separate the egg whites from the yolks. Beat the egg whites to snow and add the sugar. Keep beating and add the yolks. Next add the flour, now stirring with a spatula.
Grate the apples in a bowl. Add the cinnamon and stir. Lay out a tray with baking paper. Squeeze out the apples well and arrange them in the tray.
Pour the sponge cake batter over them but do set aside a small part of it to mix with the cocoa, use that for decoration. Bake at 356°F (180 °C) for 15-20 min.
Take the cake layer out of the tray. Invert it and place it with its baking paper onto a moist towel. Wrap it up into a roll. Leave to cool. Unroll it, remove the paper and wrap into a roll again. Sprinkle with powdered sugar.