Preheat the oven to 200℃. Combine the cake flour and cocoa powder in a plastic bag and shake well.
Put the eggs in a bowl, add the granulated sugar, and mix lightly. Warm the mixture up in a double boiler until it feels warm to the touch.
Whip well with a hand mixer at high speed for about 5 minutes. Until the batter becomes thick and drips like ribbons, leaving tracks on the batter and some mixture stays in the wire beaters.
Reduce the speed and beat on the lowest setting for about 2 minutes.
Sift in the cake flour and cocoa powder together.
Mix the batter quickly until it's no longer floury. Mix as if to scatter the flour around so as not to create any lumps.
Warm the milk in a microwave until it starts to steam lightly. Pour the warmed milk into the batter and mix well by scooping it up from the bottom, about 50 times.
Lay two layers of parchment paper in a baking tray, and pour the batter all at once into it.
Smooth out the surface with a scraper, tap it on the counter to release air bubbles. Lower the oven temperature to 190℃ and bake for 10~12 minutes.
When it's baked, place it on a wooden board and cover with one sheet of parchment paper. Put a moist tea towel on top and let it cool down.
Add the sugar to the heavy cream and whip until the cream forms a thick ribbon when the whisk is lifted. Roll the cake with the whipped cream inside, wrap with cling film and chill in the fridge for more than 1 hour. And it's done.
Please take a look at the detailed instructions for rolling a cake, Recipe ID: 829853. If you find it hard to roll the cake as there is too much whipped cream on it, reduce the amount of cream to roll.
Story Behind this Recipe
I wanted to eat a rich roll cake that would melt in your mouth. I experimented several times and finally came up with this recipe.
This sponge cake has cocoa powder inside, so it loses its airy texture very easily. So mix the batter as quick as possible after you add the cocoa powder. However, if you don't mix enough after you add the milk, the sponge will be too fluffy and crack when you roll it. Make sure to put two layers of parchment paper in the baking tray.