Preheat the oven to 330°F (165°C). Using softened butter, grease the bottom and the sides of the 8-inch (20 cm) springform pan and line it with parchment paper. Alternatively, use a cake ring where the base is covered with aluminum foil. Place the cake ring in the middle of the piece of aluminum foil and fold the foil toward the ring. Then butter and line the bottom and sides of the dish with the parchment described above. Place it on a baking tray.

Break the dark chocolate into small pieces and place in a heat-proof bowl. Add the butter cut into small cubes, melt in a double boiler or bain-marie, and set aside to cool. You can use the microwave with settings at 30-second bursts.

In a separate bowl, place eggs with sugar. Whip with an electric whisk until doubled and frothy for about 5 minutes. Add the melted chocolate mixture to the eggs and sugar preparation and mix with a rubber spatula. Ensure the chocolate mixture is cooled down to prevent cooking the eggs. Add the sifted flour and gently mix.

Pour the fondant mixture into the prepared mold and bake for 22 minutes. Remove the cake from the oven and let it cool at room temperature for 30 minutes. Then, refrigerate it for at least 3 hours, unmold it, and transfer it to a serving plate.

Note: When the cake comes out of the oven, it seems undercooked with a slightly wobbly center, which is normal. If you bake it in a ring, use the baking tray beneath your cake ring when transferring it to the fridge.

To serve, bring the cake to room temperature for 1 hour. Then, dust the top of the cake with icing (powdered) sugar or unsweetened cocoa powder.