Leave out the eggs at room temperature. Preheat the oven to 355°F/180°C. Line a pan with baking paper.
Combine and shift the flour and cocoa powder several times.
Put eggs and sugar in a bowl and put on a bain-marie at about 140°F/60°C. Whisk with an electric mixer at high speed.
Check the temperature of the egg mixture and when it's 97°F/36°C. It should feel a little warm to the touch, then remove from the bain-marie and continue to whisk.
After the mixture is fluffy (about 5 minutes after the bain-marie) reduce the speed and whisk slowly for about 3 minutes.
Whisk until the batter looks like the photo. It looks shinier than when you whisked at high speed. In this photo you might not see much difference, though.
Change to a whisk. Add 1 tablespoon of water and whisk gently by adding water, the flour will mix in easily.
Add the prepared flour in one go and mix. The oil from the cocoa powder eliminates the bubbles so whisk about 35 times. It will become shiny and thick.
Pour the batter into the pan and flatten the surface with a dough scraper.
Bake in oven at 355°F/180°C for 12 minutes.
After baking, remove from the oven. Drop the pan on the cooling rack from 20 cm high to prevent shrinking.
Peel off the baking paper, wrap the spongecake with cling film and lay on a cooling rack. Put into a large plastic bag with the rack and cool down completely.
Meanwhile, make the chocolate whipped cream. Heat the heavy cream until it is just about to boil. Add chocolate and melt. Mix with a rubber spatula.
Chill Step 14 in iced water. When it is cool, whip the cream until the cream forms a thick ribbon when the whisk is lifted. It will suddenly harden when you add the chocolate, so mix slowly.
After the spongecake has cooled, slice off the edge at the far end diagonally to let the joint stick more nicely.
Spread the cream on the baked side of the spongecake. Put more cream towards your end and less cream towards the far end.
Roll up the spongecake and place the rolled spongecake with the edge facing down. Wrap the swiss roll with cling film and chill in the fridge for at least one hour.
After chilling, slice with a warmed knife. Every time you slice, clean the blade. It is the easiest way.
I decorated it a to look a little fancy. I sprinkled gold leaf sugar on top.
Story Behind this Recipe
My husband and child love chocolate, so I made a chocolate version.
You could replace half the amount of cocoa powder with black cocoa powder. If you like it to be sweet, change the amount of sugar for the dough to 70 g. When you peel off the cling film from the cooled spongecake, it might damage the surface but the cream will be on top, so don't worry.