You can read all my tips and find the quantities for other cake pan sizes in the article.
Preheat the oven to 150°C (300°F), either with static or convection oven.
Grease and line the bottom and sides of the cake pan with parchment paper, leaving the edges overlapping to make unmolding easier.
Whip the cold heavy cream into stiff peaks and set it aside in the refrigerator.
Sift together the flour, unsweetened cocoa powder, and baking powder.
In the bowl of a stand mixer, beat the eggs with the sugar and vanilla sugar (or vanilla extract or any other flavoring of your choice) for 5 minutes or longer. The mixture should triple in volume, become white, and be smooth and creamy.
Gradually fold in the sifted flour, cocoa powder, and baking powder using the paddle attachment of the stand mixer or a rubber spatula. If using a spatula, gently fold by lifting the batter from bottom to top. You will obtain a fairly firm batter.
Then, add the whipped cream in several batches, gently folding it in from the bottom up with a spatula to preserve the air incorporated.
Place a flower nail at the center of the cake pan, with the point facing upwards, then pour in the batter. The flower nail is not essential, but it helps distribute heat more evenly and ensures a uniform bake.
Smooth and level the top of the batter in the pan. Then, tap the pan several times on the countertop. Note : This trick not only helps level the surface of the molly cake, but it also brings up and releases air bubbles that could burst during baking. The result: a perfectly flat molly cake sponge on both sides, without any unsightly domes. Bake for 1 hour and 20 minutes without opening the oven, as this could cause the chocolate molly cake to collapse (like a sponge cake).
Monitor the baking from 1 hour onwards. Only open the oven at 1 hour 10 minutes to check if the top is firm. In my oven, 1 hour 20 minutes is perfect, but adjust the time according to your oven.
Once baked, turn the chocolate molly cake onto a wire rack. Wait until it's lukewarm or cold before cutting it into layers.
Preferably, once the molly cake is lukewarm, wrap it in plastic wrap and store it in the refrigerator until the next day. This will make it firmer and much easier to cut.