Rinse the prunes well. With a knife, cut each one open and take out the pits. Replace the pit with half of a walnut and close the prune.
Melt the chocolate in Bain-Marie (water bath), add salt, butter, coffee and vanilla extract. Stir until the mixture turns smooth and runny. Carefully dip the prunes in the hot chocolate and place them on a baking parchment paper to cool. When the chocolate hardens, transfer the prunes to the fridge to harden the chocolate even more. You can decorate the prunes with a drizzle of melted white chocolate. Serve cold.