Place the cake sponge in a bowl and crumble the cake either by rubbing with your hands (it’s best to use catering food gloves for this) or using a food processor.
Heat the double cream and bring it to boil.
Add the chopped up chocolate and leave to melt. If the mixture is too cold to melt, just heat it gently until all chocolate is melted. This can be done easily in a microwave.
Add the vanilla extract and the pinch of salt to the cream mixture and blend in. When nearly cold, add the rum or rum essence and then add into the cake crumbs and mix well till evenly combined.
Using your catering gloves again, take about a large teaspoon of the mixture together and form a ball.
While the cake mixture is still sticky, roll in the chocolate vermicelli, making sure that the whole truffle is nicely covered.
Repeat with the remaining mixture.
Refrigerate for 30 minutes until firm and serve straightaway.