In a bowl, crush the cake and the digestives. A few small pieces are okay.
Mix in the cocoa powder and butter
Melt the chocolate over a double boiler or in a microwave and add it to the cake mixture. mix well.
Start adding the rum – a little at a time, and add more as needed till you reach a consistency to form a ball when rolled. If its too soft, add more digestive crumbs or cake to adjust.
Scoop the dough and roll into roughly 1 inch balls or by weight of 20-25g each and place on a parchment lined tray. Put away in the refrigerator for at least 15 minutes.
If coating with melted chocolate - Dip each cooled ball in the melted chocolate and gently place on a wired rack with the help of a fork. Garnish with sprinkles or white chocolate if you want. Repeat with all the balls and let dry then transfer to an airtight container.
Or simply dust the balls with cocoa powder.
Keep refrigerated in an air tight container till needed.