Preheat oven to 350°

1. Add bittersweet chocolate and butter in a mixing bowl
2. Place bowl over a bain marie (water bath) stirring occasionally until fully melted
3. Crack eggs, separating yolks and white
4. Pour sugar into bowl with egg yolks
5. Whip the egg yolk mixture in a stand mixer with whisk attachment
6. Meanwhile spray cake mold with vegetable oil and coat with sugar
7. Whip egg whites with whisk attachment, add in some sugar and then salt. Increase speed and gradually add the rest of the sugar
8. Add 1/3 of egg yolks to the chocolate, stirring and folding it helping lighten the mixture. Fold rest of the egg yolks into the matter. 
9. Add 1/3 of egg whites to chocolate, stir and fold. Fold the remaining egg whites into the mixture
10. Once mixed, pour batter into cake mold
11. Bake in oven for 45 minutes. Rotate cake midway through baking
12. Plate and serve

Notes

1. You can use a hand mixer to whip the egg yolks and whites if you don't have a stand mixer
2. You can use a removable bottom cake pan in place of pastry ring