Place pudding or fruit cake in the bowl of a food processor and process until finely crumbled. Turn into a large mixing bowl and add the melted dark chocolate and liqueur (if using) and mix until combined.
Roll tablespoons of the mixture into balls (20g is a good size) and place on a tray lined with baking paper. Set aside whilst you melt the chocolate.
To melt the chocolate, either place it in a heatproof bowl over a pan of simmering water, stirring often until melted and smooth. Or use the microwave method: place the chocolate in a microwave-safe bowl and heat on high power for 1 minute, stir. Return to the microwave and heat on high for 30-second bursts, stirring after each burst, until mostly melted but with some chunks still visible. Continue to stir until the chocolate is fully melted.
Using a fork, skewer or chocolate dipping tool, dip balls into chocolate to cover, then allow excess to drip back into the bowl. Return coated balls to the tray and place in the fridge to allow chocolate to set.
Spoon or pipe white chocolate over the top of the puddings (to resemble custard), decorate with a slice of cranberry and a sprinkling of pistachios to complete the look. Return to the fridge to set and keep in an airtight container in the fridge.