Grease an 8-inches (20 cm) springform pan with a non-stick cooking spray or butter. Line the bottom of the pan with parchment paper.

Over a bain-marie (or a pot of simmering water), melt chocolate and butter. Stir until smooth and combined. Set aside to cool.

With an electric mixer running on high speed, whip eggs, sugar, and salt until thick and lightened in color.

Combine the remaining eggs with the chocolate. Gently fold just until combined.

Transfer the batter into the prepared pan. Bake in a preheated oven at 325°F (160°C) for 40 minutes. Run a spatula along the sides, then remove the cake from its pan. The cake will naturally sink and create some cracks on the crust.