• Melt the butter in a saucepan over low heat. Add the chopped chocolate and stir until melted. Turn off the heat and let it cool for at least 20 minutes, stirring to speed up cooling.
• When the chocolate is lukewarm, preheat the oven to 180C (350F) on the conventional setting.
• Separate the egg yolks from the whites, then beat the whites until stiff peaks form with an electric mixer. Set aside.
• In another bowl, beat the egg yolks with the sugar using a whisk.
• Add the flour and salt all at once to the yolks, then whisk to combine.
• Add half of the egg whites to the cream, then gently fold them in with a spatula.
• Add the melted chocolate to the mixture in three stages. Each time you pour in the melted chocolate, gently stir with the spatula to incorporate.
• Finally, add the remaining beaten egg whites and mix with the spatula.
• Grease the cake tin or line it with parchment paper, then pour in the mixture and bake for 25 minutes. Then turn off the oven, open the door almost all the way, and let the cake rest for 5 minutes before removing it to cool completely.
• Dust the cake with powdered sugar and serve with whipped cream or Chantilly cream. The cake should have a crust on the surface that breaks when you touch it. That's fine.

Tips

• If you can, make it the day before! It will be better.
• Store leftover slices in the refrigerator for a few days.