Preheat oven to 350°F (180°C). Grease a 9-inch (22 cm) cake pan and line the bottom with parchment paper.
Melt the dark chocolate and butter in a heatproof bowl set over a simmering pot of water (double boiler). Stir until smooth.
In a bowl, whisk the eggs, powdered sugar, and vanilla sugar until light and fluffy.
Stir in the chestnut puree, ground walnuts, rum, and salt. Gradually add the melted chocolate mixture, stirring until just combined.
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool completely in the pan. Meanwhile, prepare the truffle cream: heat the heavy cream in a saucepan until almost simmering. Pour over the finely chopped dark chocolate. Stir until smooth and glossy.
Let the chocolate cream cool slightly, then spread it evenly over the top and sides of the cake.
Dust with cocoa powder and garnish with chocolate shavings or grated chestnut.
Refrigerate for at least 2 hours to allow the cream to set properly.
Slice and serve, and enjoy the rich, chestnut-chocolate truffle flavor!
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