Place the tim tam biscuits in a large zip-lock bag and gently crush with a rolling pin until crumbly, leaving a few small chunks for texture.
Beat the cream cheese until soft and smooth. Add the crushed biscuits and fold together with a wooden spoon until well combined. Don't overmix.
Refrigerate the mixture for 15–20 minutes to firm slightly.
Scoop out dessert spoon-sized portions and roll between your hands to form balls. Place onto a baking paper-lined tray and refrigerate for another hour, or until firm.
Melt the dark chocolate using either a bain-marie or microwave. Bain-marie: Place the chocolate in a heatproof bowl over a saucepan of gently simmering water, making sure the bottom of the bowl doesn't touch the water. Stir until smooth. Microwave: Heat in short bursts, stirring thoroughly between each one, until melted and smooth.
Using two forks, dip each chilled ball into the melted chocolate and turn until completely coated. Lift out, allow the excess chocolate to drip away, then return to the lined tray.
Add the festive sprinkles straight away before the chocolate sets.
Refrigerate until completely firm, then transfer to an airtight container and keep chilled until ready to serve.