Put the cream and dark chocolate in a microwave-safe bowl and microwave at 900W for 1 minute 30 seconds, stirring every 30 seconds. Remove and stir until smooth. Leave to cool, cover with clingfilm and leave to set in the fridge for 2-3 hours, until firm.
Remove from the fridge and leave to soften for 20 minutes. Line a baking tray with baking paper. Using a teaspoon, scoop out spoonfuls of the mixture, roll into balls and place on the tray.
Use a fork to dip and coat the truffles in the melted milk chocolate, shaking off the excess. Put in the fridge to set. Once set, drizzle over the melted white chocolate in a zigzag pattern. Leave to set, then transfer to petit-four cases.