Preheat your oven to 180°C (350°F) and grease your baking pan to prevent sticking.
Combine the chopped dark chocolate and butter in a heatproof bowl, then melt them together over a double boiler or in short bursts in the microwave, stirring until smooth and glossy. Let it cool slightly.
Pour the melted chocolate mixture into a large mixing bowl, then add the sugar and whisk until well combined and shiny.
Add the eggs one at a time, whisking vigorously after each addition until the batter is glossy and slightly thickened.
Stir in the vanilla extract, ensuring the mixture is smooth and uniform.
Sift in the flour and salt, then gently fold them into the batter using a spatula, taking care not to overmix. The batter should be thick and fudgy.
Pour the batter into the prepared baking pan, smoothing the top with a spatula. It will look dense and shiny.
Bake in the preheated oven for about 25 minutes, or until the top is cracked and the center is just set but still fudgy when tested with a toothpick.
Remove from the oven and allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack and cool completely before slicing.
Slice the cake with a sharp knife, revealing the dense, moist crumb with tiny melted pockets of chocolate inside.
Serve the cake as is or with a dusting of powdered sugar for a simple, indulgent dessert.
