Preheat the oven to 150°C (302°F) in conventional oven or fan-assisted mode.
Grease and line bottom and sides of the pan with parchment paper. Cut the parchment paper higher than the cake pan.
Whip the very cold heavy cream until firm but supple. Set aside in the fridge.
Sift the flour with the baking powder.
Beat the eggs with the sugar and vanilla sugar (you can use vanilla extract or other flavorings) for 5 minutes or more. The mixture should triple in volume, becoming white and creamy.
Add in the flour and baking powder mixture in batches into the bowl of the stand mixer or with a rubber spatula If using a spatula, work gently from bottom to top. The result is quite firm.
Add a little whipping cream and mix gently. Gently add the rest in batches, working from bottom to top.
Place the flower nail in the center of the pan, with the pointed side facing up, and pour the batter.
The flower nail is not essential but helps distribute the heat and achieve an even bake.
Level and smooth the top.
Tap the mold against the work surface several times. This can help you to even out the surface and avoids a bump. you will get a flat molly cake sponge on both sides.
Bake for 1 hour 20 minutes. Do not open the oven to avoid the molly cake collapsing (same principle as with sponge cake). Monitor the baking from 1 hour and only open the oven at 1 hour 10 minutes to see if it's firm on top.
For me, after 1 hour 20 minutes, it was perfect. It's up to you to estimate how long this might take for your oven based on your previous experiences.
Once baked, turn it onto a wire rack and let it cool.
Cut into layers only when warm or cold.