Begin by melting the bittersweet chocolate and butter together in a heatproof bowl set over simmering water, stirring until smooth and glossy. Let it cool slightly so it doesn't scramble the eggs.
In a separate large bowl, whisk the eggs, sugar, vanilla, and salt until the mixture becomes light and slightly aerated, about 2-3 minutes.
Pour the slightly cooled chocolate mixture into the egg mixture and gently fold together until fully combined and smooth.
Line your baking dish or loaf pan with parchment paper or lightly grease it. Pour the chocolate batter into the prepared dish, smoothing the top with a spatula.
Bake in a preheated oven at 350°F (175°C) for about 8-10 minutes. The edges should be set, but the center will remain soft and gooey.
Remove from the oven and let the cake cool in the pan for 10 minutes, allowing the center to thicken slightly but stay deliciously fudgy.
Transfer the cake to the refrigerator and chill for at least 2 hours, or until completely cold and firm on the outside but still with a molten center.
Once chilled, slice into portions and serve. The exterior will be firm, while the interior remains rich, thick, and velvety—like a dark, chilled river of chocolate.
