Preheat your oven to 325°F and prepare an 8-inch springform pan with parchment paper.

Beat cold, large eggs at high speed in a stand mixer until doubled in volume and pale yellow.

Melt bittersweet or semisweet chocolate and unsalted butter over simmering water until smooth.

Fold a third of the whipped egg mixture into the melted chocolate to lighten it; add the remaining egg mixture gently.

Transfer the batter into the prepared pan and set it in a larger roasting pan with boiling water half up the sides.

Bake for 22-25 minutes until edges are set and a thin crust forms on top, center should be soft.

Cool the cake at room temperature for an hour, then refrigerate overnight.

Remove the sides of the springform pan and dust with sugar or cocoa powder before serving.