Leave out the eggs at room temperature. Preheat the oven to 180℃ (350℉). Line the pan with baking paper. Soak the gelatin and set aside.
Sift the white flour 2 to 3 times.
Add eggs and sugar in a bowl and put on a water bath at about 60℃ (140℉). Whisk with an electric hand mixer at high speed.
Check the temperature of the egg mixture and when it's 36℃ (97℉) (it should feel a little warm when you touch), remove from the warm water bath and continue to whisk at high speed.
After the mixture starts to thicken (about 5 minutes after the warm water bath) reduce the speed and whisk slowly for about 3 minutes.
Whisk until the batter looks like the photo. It looks glossier than when you whisked at high speed. It may be a bit difficult to see in the photo.
Change to a whisk at this point. Add 1 tablespoon of water and whisk gently. By adding water, the flour will blend in easily.
Add the prepared flour in at one time and whisk about 40 times until glossy and thick.
Pour in the batter from above into the pan and smooth the surface with a dough scraper.
Bake in oven at 180℃ (350℉) for 12 minutes.
Once baked, remove the pan from the oven. Place the pan on the cooling rack about 20 cm high to prevent shrinking.
Peel off the baking paper, tightly wrap the cake with plastic wrap and lay on a cooling rack. Put the cake and cooling rack into a large plastic bag and cool down completely.
Meanwhile, make the cream filling. Whip the heavy cream until it's thickened (i.e. until you see a small amount of tracks on surface when whisk is lifted). Cut the fruit and set aside.
Using the same electric hand mixer, mix the egg yolks and sugar while warming in a water bath until they whiten and thicken.
Warm the soaked gelatin on a water bath or in the microwave. Do not let it boil! It will become clumpy if you let it boil. Add to the mixture from step 15 and mix.
Combine and blend the mixture from step 14 and 16 together. The cream filling is done.
Once the sponge cake has cooled, slice off the edge at the far end diagonally to let the edges stick more nicely.
Spread the cream on the baked side of the sponge. Put more cream towards the close end and less cream towards the far end.
Arrange your fruits of choice and roll.
Place the rolled sponge cake with the edges faced down. Wrap with plastic wrap and chill in the fridge for at least an hour.
After chilling, slice the roll cake with a warmed knife. If you warm your knife and clean it every time, you can slice it very nicely.
Moist Fluffy Plain Swiss Rolls: Chocolate version Recipe ID: 1028702 & Matcha version (Recipe ID: 1029067)
Story Behind this Recipe
I wanted to make a moist Roll cake batter, and after having failed many times, I finally came up with a good roll cake. I adapted the cream filling to my taste from a cookbook.
This cream filling contains gelatin, which makes it very speedy to cook from step 19. Even if the baked surface of the batter gets spotty when removing the plastic wrap once cooled, it's not an issue since you're spreading cream over it.