Preheat your oven to 350°F (180°C) and line a small baking pan with parchment paper.
Melt the dark chocolate and butter in a heatproof bowl set over a pan of simmering water (or in the microwave). Let it cool slightly.
In a bowl, whisk together the sugar, vanilla sugar, and eggs until pale and frothy.
Add the pinch of salt and the chocolate mixture, then stir until smooth.
Sift in the flour and cocoa powder, and gently fold into the batter until just combined.
Pour the batter into the prepared baking pan and spread evenly.
Bake for 20-25 minutes, or until the top is set but the center remains slightly fudgy.
Let cool completely in the pan, then cut into squares and serve.
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