Procedure

Put the chocolate in a heatproof bowl.
Put cream and butter in a small pan. Heat and mix until the butter starts to melt and the mixture in just boiling.
Pour the hot mixture on the chocolate. Mix until the chocolate melt and the texture is smooth.
Add the rum and the cake crumbs while mixing. Put in the fridge for 20 minutes. Roll the mixture into truffles.
On a sheet of baking paper, spread the dark cocoa powder and roll each balls in.
Line the truffles on a baking tray with foil. Refrigerate for 40 minutes.
Can be kept in an airtight container in the fridge for up to 1 week