Melt dark chocolate and butter together in a double boiler.
Add sugar to the melted chocolate and mix well.
Stir in the eggs thoroughly.
Sift cake flour into the mixture and fold gently.
Pour the batter into a mold.
Bake in a preheated oven at 180°C for 10-12 minutes.
Take out the finished fondant au chocolat, let it cool, and plate.