Preheat oven to 320°F (160°C). Grease a 20 cm cake pan and line the bottom with parchment paper.
Melt the dark chocolate and butter in a heatproof bowl set over a pan of simmering water (or in the microwave in 30-second intervals), stirring until smooth.
In a separate bowl, whisk the eggs, granulated sugar, and vanilla sugar together until light and fluffy.
Sift in the cocoa powder, flour, and salt. Gently fold into the egg mixture until just combined.
Pour in the melted chocolate mixture and stir until a smooth batter forms.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until the edges are set but the center is still slightly soft.
Let the cake cool completely in the pan. To make the truffle topping, heat the heavy cream in a saucepan until simmering. Pour the hot cream over the finely chopped dark chocolate. Let it sit for a minute, then stir until smooth. Let it cool slightly.
Pour the truffle topping over the cooled cake and spread evenly.
Refrigerate for at least 2 hours to allow the truffle topping to set.
Dust with powdered sugar, cocoa powder, or chocolate shavings before slicing and serving.
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